The Deepest Depths of Resentment is a bonus dungeon added into Souzinengi V as a replacement for the Evil Map present in Weekend. As a result, it wasn't available to english speaking players until now and, thus, there's effectively no english info on the dungeon available. Unlike the previous bonus dungeons, though, it's long and arduous, and you're certainly not making it through unless you've already AT LEAST raised the level cap. Make sure you're sufficiently prepared before tackling this challenge.

Note: Thank you to ZeldaFan024 for providing the exact rooms each enemy can be found in. You can check out their guide on that here: https://steamproxy.com/sharedfiles/filedetails/?id=3038934024

For those who are unaware, the Evil Map was a notoriously difficult bonus dungeon in previous releases of the game. The dungeon layout itself was rather uninteresting, and instead, it was just about fighting your way through enemy encounters that couldn't be fled from. Most of the enemies were pulled from Depths of Resentment with a few from Gorgon Sanctuary, and the final few rooms were an absolute gauntlet where you had to fight the random encounter midbosses from both aforementioned maps. It was a hellish dungeon where you absolutely had to be at the level cap and use all your resources to their fullest potential, typically reserving Bombs for only the final few rooms and using as little MP as possible in all the rest. However, there was one saving grace in that the enemies followed special rules for respawning, which you could abuse to make it through more easily.

This dungeon isn't like that. Technically, it's longer (12 rooms instead of 8), but you can actually flee from the encounters for most of the dungeon. Additionally, the dungeon is populated with all new enemies, and there's even a brand new boss fight at the end to top it all off. It's all still rather difficult, and the enemies actually respawn normally whenever you leave, but it's much more reasonable than the original Evil Map.

To unlock the Deepest Depths, all you need to do is challenge the Resentment Incident, the final boss of Lingering Summer Heat. You don't have to win, so once you've challenged it, just return to the shrine and accept the quest "Deepest Depths of Resentment," and a new teleporter will appear along the south edge of the save room at the end of Depths of Resentment.
Note: While this dungeon can be attempted as soon as it's available, I highly recommend actually defeating the Resentment Incident to unlock some very helpful synthesis recipes and clearing at least the Library Depths to raise the level cap. Attempting this dungeon while still capped at 70 is just asking for trouble.

The Deepest Depths are split into two distinct segments, which I'll be dubbing the Upper Deepest Depths and the Deepest Deepest Depths. The Upper Deepest Depths comprises the first 8 rooms and features a mostly linear path with brief side paths leading to loot and switches to raise bridges along the main path. Unfortunately, I was unable to properly map out the dungeon as I felt myself getting dizzy whenever I tried to use the floor spirals to get any sense of scale, but the dungeon's linear enough that you should be able to make it through with just the written guide and screenshots.

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Starting in room A, follow the path around the right side of the room to loop around to the left exit.
Pass through this door into room B and hit the switch to raise a bridge on the main path.
Return to the start of room A, and this time, follow the path along the left side of the room to drop down to the lower level by the middle exit.
Pass through this door into room C, then pass all the way through room C to find yourself on the lower section of room B. Instead of making your way to the bridge you lowered, first head to the middle exit.
Pass through here into the lower section of room D and then hit the switch to raise another bridge on the main path.
Now, return to room B, take the stairs back onto the upper section, and take the bridge to the right exit.
Pass through the door into room D, follow the path to the new bridge and the only other exit into room E, and then continue south through room E until you reach the bottom of the stairs. Before passing into room F on the left, take a detour to the right to hit a somewhat hidden switch and raise a bridge in the next part of the room.
Then pass into room F on your left and follow the path through to an exit back into room E, this time on the other side. Instead of taking the stairs back onto the upper portion, head south to the right exit.
Pass into room G, then hit the final switch to lower the final bridge.
Now return to room E, take the stairs onto the upper portion, and then cross the bridge to the left exit.
Pass into room G, and then follow the path down the final bridge and into room H. Walk along the right path to loop into the center of the room and then WALK AROUND the healing circle to reach the teleporter to room I.

The rest of the dungeon from room I onwards comprises what I call the Deepest Deepest Depths and consists of three rooms and a boss room. From this point onwards, you can no longer flee from enemy encounters. Whatever you fight, you must commit to, and as the encounters are still randomly selected, you must be prepared for anything. Before progressing any further, do the following:

1 - Make sure all party members you plan to use are at a BARE MINIMUM of level 75 and have at least their Ostracon tier weapons, though Orichalcum and especially Super Ajan tier are preferable.
2 - Leave the dungeon entirely, then equip any Bomb or MP increasing equipment before coming back.
3 - Avoid all encounters on the way back and walk around the one-use healing circle so as not to use it.

Recommended party members:
Alice - A mainstay for any party. Hourai Doll and Little Legion are incredibly valuable skills, and Marionette/Rainbow Parar are great for if you're willing to put most of your party in one column.
Patchouli - Give her an Awakened Soul for lots of Bombs, then let loose with whichever elemental spell best fits the situation. Getting off a double cast will chunk most enemies here, especially if you hit a weakness.
Sanae - Her buffs are incredibly helpful, especially Wind of Miracles, and healing and revives are a godsend to have on hand.
Marisa - Her personal equipment can give her a healthy Bomb pool, letting you use Ultimate Spark at the start of most fights. A double casted Ultimate Spark shreds nearly everything you'll run into.
Litos - Go all in on Bomb and MP equipment, then start as many fights as you can with Cosmic Supercluster to wipe out a solid chunk of HP (or outright kill weaker enemies) right at the start.
Reisen - Many enemies here can be debuffed or statused easily, leading to massive damage from her Slayer effects.
Sakuya - Quick Time can trivialize effectively any encounter, so long as you have the MP and Bombs to spare, though her low Acc and MP may make her otherwise tricky to use.
Youmu - Tricky to use against enemies that can stun, but she can shred most encounters single handedly if given time to set up.
Aya - Being able to guarantee everyone moves first is incredibly helpful in emergencies, and guaranteeing your physical attackers don't miss can be a godsend. Beyond that, though, she's not much use.

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Room I - 0 Mandatory Encounters
This room is largely a non-issue as the couple enemies on your path can be easily dodged. Take the teleporter on the right (the other one leads to an immediate dead end and spawns you on top of an encounter).
Then follow the path along to the exit. Both encounters can be easily avoided if you pay attention, though the second will likely get you if you go for the chest.

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Room J - 6 Mandatory Encounters
From this point, enemies will block the path forwards and can not be avoided. Only fight what you have to.

Possible Encounters:
Predator Dragon x2
Top Ace, Hraesvelgr of Death, Resentment Silkmoth
Tower of Hell, Tower of Ice, High Ninja, High Lich, Tsukuyomi
Top Ace, Solar Luminescence, Patriotic Dogazer
Minotauro Zombie, Leopard II, Parapuzosia
Farora, High Ninja, Emperor Cockatrice
Fairy Princess, Emperor Cockatrice, Death Mushroom, Midnight
Vassago
Andrealphus
Yukkuri Emperor
Yukkuri Emperor, Reimu Emperor

Fight #1 is directly in front of the entrance teleporter.
Fight #2 is off to the left.
Fight #3 is around the bend and to the right.
Fight #4 is directly beyond the last fight.
Fight #5 is down to the right. There's another fight to the left that just leads to a dead end.
Fight #6 is directly in front of the teleporter off to the right. There's an additional encounter blocking a teleporter to the left that leads to a single chest (and then a dead end) and another encounter blocking nothing.

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Room K - 8 Mandatory Fights

Possible Encounters:
Vassago, Resentment Silkmoth, Tower of Gaia
Andrealphus, Patriotic Dogazer, Tower of Babel
Predator Dragon, Ogre Mage, Midnight
Tsukuyomi, Resentment Silkmoth, Elite Dullahan
Crystal Wyvern, Hraesvelgr of Death
G Kinzamu
Top Ace, High Lich, Emperor Cockatrice
Solar Luminescence, Emperor Cockatrice, Lonesome George
Crystal Wyvern, Farora, Death Mushroom, G Kinzamu, Tower of Babel
Yukkuri Emperor
Yukkuri Emperor, Reimu Emperor
Yukkuri Emperor x2, Reimu Emperor

Fight #1 is once again directly in front of the entrance teleporter.
Fight #2 is directly off to the left.
Fight #3 is around the bend and to the right. There's an additional fight further to the right that leads to a dead end.
Fight #4 is directly ahead of the last one.
Fight #5 is directly ahead once again.
Fight #6 is off to the right.
Fight #7 is off to the right again.
Fight #8 is directly in front of the teleporter to the boss room. There's another encounter further to the right that leads to a dead end.

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Room L - The Safe Room
Once you're here, flip that switch to activate the shortcut teleporter and take a break. You've earned it.

1 - Chrono Armor - A
Decent heavy armor that provides potentially the only method of stopping specifically Stop.

2 - Divine Spirit Shard - A

3 - Platinum Shard - A

4 - Giant Buster - B (Right entrance)
A powerful sword with an absurd level of Giant Slayer. Great for certain quests.

5 - Barrier Fragment - B (Left entrance)

6 - Vajra Robe - C
Great light armor with an even bigger HP buff than the Perfect Barrier Suit.

7 - Barrier Fragment - C

8 - Hell Armor - B (Middle entrance)
Being immune to Slayer effects would be great if we knew what had Slayer effects outside Satori's and Akyuu's skills. Healing statuses faster is great, though.

9 - Ostracon - B (Middle entrance)

10 - Silver Mimic(!!) - B (Middle entrance)

11 - Abyss Shield - D (Left entrance)
Decent medium physical shield.

12 - Flower - D (Left entrance)

13 - Ring of Alloces - D (Right entrance)
Accessory that increases ETH resist by 88%

14 - Divine Spirit Shard - D (Right entrance)

15 - Barrier Fragment - D (Right entrance)

16 - Great Philosopher's Staff - E (Left side)
Decent staff that increases cast speed. Probably not gonna see too much use, though.

17 - Gold Mimic(!!!) - F

18 - Angel Battleaxe - F
Powerful axe that's great for killing demons.

19 - Ring of Forneus - E (Right side)
Accessory that increases WAT resist by 88%

20 - Platinum Shard - E (Right side)

21 - Divine Spirit Shard - G (Right entrance)

22 - Ring of Haborym - G (Left entrance)
Accessory that increases FIR resist by 88%

23 - Ring of Bune - G (Left entrance)
Accessory that increases THU resist by 88%

24 - Fairy King Spear - G (Left entrance)
Spear that increases your Pow gain and Res. A decent choice to equip if you want to grind Pow levels.

25 - Platinum Mimic(!?) - H

26 - Platinum Shard - H

27 - Killer Maneater - H
A decent knife that can inflict Confusion and has a good Slayer effect.

28 - Barrier Stone - I

29 - Platinum Ore - J

30 - Divine Spirit Stone - J
(I apparently never screenshotted this one and only just found out as I was writing this guide. Whoops)

31 - Recipes - K (Wrong teleporter)
Here are the recipes for the Barrier Stone/Platinum Ore tier weapons. They're upgrades to the Ostracon tier and have slightly less power than the Super Ajan tier, though their effects are on par and great alternatives if you don't wanna grind the resources (or just prefer a different effect).

32 - Black Diamond - K (Right teleporter)

33 - Super Ajan - K (Right teleporter)

Tower of Hell//Babel/Gaia/Ice
The Towers from the Library and Temple Depths are back, this time with a new friend. They're annoying support enemies that strengthen and attack with their elements while spamming Shield Wall and Imposing Stance.
Found In - (Hell) A, C, I, J ; (Babel) A, B, K ; (Gaia) A, B, C, D, E, K ; (Ice) A, B, C, D, E, G, I, J

Death Mushroom
Relatively easy to deal with, though they can debuff your stats and inflict Severe Poison and Death.
Found In - A, B, C, D, E, G, H, I, J, K

Resentment Silkmoth
Pretty easy to deal with, though they're priority targets as they can inflict party wide Lethal Poison.
Found In - A, B, C, D, E, F, H, I, J, K

Minotauro Zombie
Powerful physical attacks, though that's about it.
Found In - A, B, C, D, E, F, H, I, J

Elite Dullahan
Dangerous enemies that love to inflict Death. Kill them before they kill you.
Found In - A, B, C, D, E, F, G, H, K

Fairy Princess
Surprisingly dangerous, given Ribbon Slice, though they're also incredibly frail. 10% chance to drop the Crimson Knife.
Decent damage knife with a lot of HP drain.
Found In - A, B, C, D, F, H, I, J

Lonesome George
Lonesome George is unique in that it gets two actions and will usually spend the first one on Imposing Stance. It also gets Hyper Fang Crush, a skill otherwise locked to boss fights. 10% chance to drop the Diamond Turtleshell Shield.
Decent medium magic shield.
Found In - B, C, D, G, H, K

Emperor Cockatrice
COCKATRICE! KILL IT NOW! Party wide Petrify as usual. 10% chance to drop the Ring of Dexterity V, which gives +20 DEX.
Found In - C, D, F, G, H, I, J, K

Leopard II
The least notable enemy in the dungeon. It's just a slightly stronger version of the generally non-threatening tank cats we've seen so far.
Found In - C, D, F, G, H, I, J

Crystal Wyvern
Massive ice dragon. Dangerous if left alone. 10% chance to drop the Ring of Strength V, which gives +20 STR.
Found In - C, E, F, G, H, K

Parapuzosia
A stronger version of the Ammonite, an already dangerous enemy. Kill it fast or find a way to deal with its breath attacks.
Found In - B, D, E, F, G, H, I, J

Patriotic Dogazer
Incredibly annoying enemy that inflicts Control and Stop.
Found In - D, E, F, G, H, I, J, K

Farora
Sets land to a random element on the first turn, then will either waste a turn setting land again or use A LEVEL 70+ PATCHOULI SKILL OF THAT ELEMENT. Thankfully, they're rather frail. 10% chance to drop the Magus Wand.
Decent staff that increases MDef by A Lot. A great defensive tool for a weapon.
Found In - D, E, G, H, I, J, K
Fun fact! This enemy was designed by one of four winners of one of two official Touhou Card Monsters tournaments.

High Ninja
Highly evasive enemy that can inflict Death and Severe Poison. Not a problem if you have magic ready. 10% chance to drop the Ring of Agility V, which gives +20 AGI.
Found In - E, F, G, H, I, J

Vampire
Can inflict Charm and drains HP. Not too notable otherwise. 10% chance to drop the Blanc Neige Licorne.
Decent spear that increases MDef by quite a bit. A good defensive tool for a weapon.
Found In - E, F, G, H

Hraesvelgr of Death
A bulky evasive enemy with TWO DIFFERENT PARTY WIDE EVASION BUFFS that can also inflict Control, Death, and Zombie. Use magic or Aya's Tengu's Fall Wind. 10% chance to drop the Titan Suit.
Powerful heavy armor that increases Max HP by a whopping 25%. Very helpful for anyone but Alice, who hit the HP cap naturally well before this point.
Found In - F, G, H, I, J, K

Ogre Mage
An Ogre variant with magic. Almost as unnontable as the Leopard II.
Found In - F, G, K

High Lich
A stronger version of the Lichs from Hieda Manor Basement. Tons of resistances, no weaknesses, and a wide variety of magic. A pain in the neck to deal with. 10% chance of dropping Orna's Magic Blade.
Powerful sword that gives a MASSIVE MDef buff and a decent Res buff. Very good defensive and offensive tool.
Found In - G, H, I, J, K

Predator Dragon
Powerful dragon that can use Gather Strength to lead into Headfirst Charge or Inhale to lead into powerful breath attacks. 15% chance to drop the Elder Dragon Dress.
Really good stats for small armor.
Found In - G, I, J, K

Top Ace
Not to be confused with the enemy with the exact same name and sprite found in the Underground, though they work just the same, spamming Light attacks, though they also have access to Starlight Barrier now.
Found In - G, I, J, K

Midnight
KILL IT NOW! They can inflict Death and LOVE to use Diabolic Breath. Can easily party wipe you on its own. 15% chance to drop the Ring of Potential V, which gives +20 POT.
Found In - H, I, J, K

Tsukuyomi
Incredibly annoying and bulky support enemy that can use Reflect Wall and various party wide buffs.
Found In - H, I, J, K

Solar Luminescence
A stronger version of the Luminescence found in Hieda Manor Basement that can now inflict Berserk. 15% chance to drop the Ring of Intelligence V, which gives +20 INT.
Found In - H, I, J, K

Vassago
Very powerful Earth aligned demon that gets 2 actions per turn and can party wipe you on its own. 20% chance to drop the Ring of Vitality V, which gives +20 VIT.
Found In - I, J, K

Andrealphus
Thunder counterpart to the Vassago that's just as dangerous, if not moreso. 20% chance to drop the Cantrip Guardian.
Decent medium armor with a lot of Res.
Found In - I, J, K

G Kinzamu
Incredibly powerful yet frail machine that gets 2 actions per turn and spams powerful Slash attacks, including Ribbon Slice. High priority target. Do not let it attack. 10% chance to drop the Godarmor Crusher.
Incredibly powerful hammer with Exorcism that ignores a chunk of PDef AND debuffs PDef.
Found In - B, K

Once you feel sufficiently rested, head to the north edge of room L to confront the boss.
Chronos is a MONSTER, and he'll shred your party in seconds if you let your guard down, which is terrifying when you consider his titanic 300k HP pool and Unknown species, which makes him immune to Slayer effects. He gets 3-4 actions per turn and loves Stone Shower and Time Stop, and he has access to a multitude of incredibly powerful Slash and Strike attacks. He also has access to Flash, Hyper Armor, Dispel, and Vortex. If you get stopped by Time Stop, he'll very often lead into the next turn with God Giant's Roar, which has the worst accuracy in the world, but anyone inflicted with Stop can't dodge it and WILL die. His whole kit, including that 1200 PDef, completely invalidates physical attackers, but his physical attacks will pulverize any of your magical attackers, even with Sanae's PDef, HP, and Variety buffs and Alice's Marionette Parar active.

For your party, you have a few options. Poison Shield is a necessity due to the very high chance of the decoy being targeted, though it runs the risk of the back row being annihilated by Paralyze Zapper. You absolutely need a tank, first off. Alice is the classic. Hourai Doll and Little Legion are perfect for keeping Chronos focused on Alice, though she'll need frequent healing. Litos is a decent alternative due to her high defense, inherent 50% Divine Shield, and regen, and she can even stack on some decent damage if you wanna just spam Cosmic Supercluster. The downside, though, is that she has no way to increase her odds of being targeted. Your funniest option is Alice AND Akyuu. Alternating between Little Legion and Imposing Stance will guarantee no one else gets hit ever, but you'll need to give up an extra party slot for it. Your second party member should be Sanae. She's a great healer, and her buffs being party wide means that she can actually keep up with Chronos' occasional Vortexes. Byakuren's single target buffs take too long to set up to be viable for this fight. As for damage, you want Marisa, Patchouli, and Satori. Marisa should use Stardust Stroke whenever it's off cooldown and Love Typhoon on every other turn until she runs out of Bombs, at which point she should switch to Rapidfire Master Spark and Super Magic Napalm. Patchouli should set Thunder land immediately then start alternating between Roaring Thunder and Hot Jupiter. Satori should use Gibnut Zarnica whenever buffs are active (until she runs out of Bombs) and Mad Thunder on every other turn. If running the Alice+Akyuu strat, I recommend sacking Marisa specifically. Patchouli is practically mandatory, and Satori's Thunder spells are too good to pass up. As for a commander, I personally recommend using Akyuu unless you're using her to tank. Her Healing skill can cure Bad Varieties, which is a godsend if anyone gets hit with Stop and shuts down God Giant's Roar entirely. If using Akyuu to tank, however, Mokou is a great alternative commander with her passive Endure chance and Phoenix's Tail as a quick revive.

No matter what, you're in for an incredibly long endurance match with plenty of chance for everything to go horribly wrong. If you're not in your high 70s at least, this fight will be a nightmare. Throw something on in the background, don't get discouraged, and keep pushing until he goes down.

Upon defeating Chronos, you gain access to the final chest of the dungeon, which contains various recipes.
Specifically, the Soli Deo, That Which Protects All, Wastelayer Claiomh Solais, Plasma Vibroknife, Carbonek Rhongomyniad, Meteo Buster, Neo Azoth, Relic Aegis, Great Mirror Shield, Evil Sealing Robe, Emerald Shield, and the tier VII stat boosting accessories. All very much worth the effort.

Additionally, you can retake this quest at any point to rematch Chronos and farm Great Orichalcos Crystals.

The true reward, though...
The final final FINAL dungeon is now open.