Obviously, this guide contains major spoilers. Also obviously, this will take away any sort of authentic experience of the various story versions, so make sure you get that out of your system before you start on this - though I can obviously never fault a fellow try-hard on their way to artificial reward points!

For random events, general outcomes and route choices, follow this walkthrough[gamefaqs.gamespot.com]. For specifics on how to do specific - especially immediate - achievements, check out this one:
https://steamproxy.com/sharedfiles/filedetails/?id=1619669160
It's really good and large amounts of my pointers about what to do where come from there. I also don't just want to copy another person's work into my guide indiscriminately. If anything in this guide is unclear, the better explanation is there. Achievements that are not discussed here in detail will be linked to whenever I remember... whenever I don't, you know what to do anyway.

In general, if you don't feel at home with the game's mechanics yet enough to get through battles fairly quickly and safely, hone those skills a bit more first.

Lastly, feel free to add or correct things in the comments - it is unlikely I will be returning to this game and guide a bunch after I've completed everything myself, but other people always benefit from additional insights! :)

In your first playthrough, you'll earn a lot of achievements. Some of the immediate ones are super easy, some are more tricky. 4 of them can be done immediately in chapter 1, before leaving Albius.
The other half earned in this run will pop after you finish the final battle. Keep as many people alive as possible, it'll make things easier. You cannot lose a single battle in this run. You'll want to start on this already knowing how to fight, but in short, you can restart fights at any time and they won’t count as lost.



General Notes

• You need to get to the White Tower as quickly as possible to save Mact (and his hand). For the fastest route, check Alina's guide under Mact makes it.
  
• This also means no resting/healing, which means no/very little wounds in battle or due to strix shortages. You will get some free party heals along the way, but not many (and only 1 is reliable, the Arch Menhir).
  
• Don’t be cruel, but don't stop to help anyone, risking extra days, don’t go looking for fights that don’t find you, but choose fighting over lengthier options like discussions or evading roadblocks. More fights = more strixes.
  
• Do not use cards in battle - they might as well not exist for this run.
  
• There are like 2 or 3 battles in which the game forces you to take Gleda. Don't worry about those, but otherwise make sure to never have her in your team, ever.
  
• optional Use the GameFAQs walkthrough to make (dialogue) choices that increase loyalty and morale unless this guide states otherwise.

Albius:

• buy the silk handkerchief from Padagang
  
• tell the Reaper "please don't hurt my daughter"
  
• talk to Tenner, your servant, ask him about his intentions,, then kill him
  
• side with Flitt over Krieger
  
• don't be rude to Fisk (or just anyone)
  
• check Alina's guide for the Not Energetic, Not a daredevil, The last word and Archers rule! achievements. It's tough to do all of them in the two battles in Albius without reloading battles, but doable.

Watchtower

• talk to Stein
  
• tell Krieger to kill Stein when you leave

Dynford Menhir

• Get here ASAP – you're follwoing the Mact makes it map anyway, that's enough – to meet and recruit Vai.

Arch Menhir

• talk to Chila at the menhir, go through all dialogue options
  
• talk to Krieger and give him the silk handkerchief
  
• Help Fisk when he gets in trouble. You’ll need some gold if you don’t want to fight.

after Arch Menhir

• When you start encountering Ense, you can get Death to the javelin thrower. You just need to kill a javelin thrower with a single blow. They can hurt themselves without affecting the achievement, but make sure they're otherwise uninjured by your team. Easier with Hopper and Lo Pheng, but doable with all 3 - I got it „on accident“.

Ursus

• go to prison by going to Unda’s house, get executed (just don’t accept anyone’s help while in prison)
  
• reload the save*, go to prison again and escape – read the Thorn’s alive! section of Alina's guide if you need help. Hode will get hurt and leave the party for now.

*This means doing the fight at Unda's place twice, but it’s more efficient than trying to squeeze the execution into a later playthrough.

(whoops, too long, had to split the first run)

General Notes

• Never ever resist the curse. Let it take over Hopper every time; you need this for the Altruist achievement and will also make life easier for Thorn and Pheng.
  
• You have to solve the riddle completely. If you miss a clue, reload.

Albius

• talk to Coronzon, exhaust the dialogue – he needs to end up owing you a favor
  
• when talking to Treeg and Coronzon, side with Coronzon
  
• You’ll get Berserker here after the fight at the Bell Tower. It's easy, just don't die.

Dead Spring

• talk to the girl, ask if she needs help, drink her water → TIP 1

Arch Menhir/Double Menhir

• talk to Vegen, ask him about Thorn
  
• check Chila's tent
  
• read the menhir’s inscription
  → INSCRIPTION 1, TIP 2

en route to Ursus via Bread Market

• ignore 1st tree
  
• investigate 2nd tree → TIP 3

Ursus

• make sure to visit Hode who got hurt earlier – you need to help him or he’ll die
  
• if you talk to Coronzon, DO NOT ask for his help* (or just don’t talk to him)
  
• talk to Teek, ask about healers
  
• go see Unda; search her house or talk to Philia → INSCRIPTION 2, TIP 4 (from Sheck)

*if you messed up the interactions with Coronzon here or in Albius somehow, you can still save the king (if you killed Tenner and Stein like you were supposed to) – you have to get rid of Sopp (Thorn’s party) by letting him die in battle. That’s a bummer, though, so the easiest way is to just talk Coronzon out of the assassination attempt later at Opacum (see below).

Scarlet Tower

• talk to Atori, offer your help (kill some bandits), return and read magic writings → TIP 5

Marsh Watch

• talk to Rask/Chirlan, exhaust dialogue → INSCRIPTION 3

Warden's Mansion

• help the warden → TIP 6

Friga

• talk to Lon → INSCRIPTION 4, TIP 7

Bear's Inn

• help out Baztitch (go find herbs), then stay and drink → TIP 8

Water Menhir

• talk to Adna → INSCRIPTION 5
  
• keep talking to her until she leaves → TIP 9

Toll Collector’s Tower

• you’ll meet Bernard: answer his question correctly (inscription locations) → TIP 10
  
• fight the bandits (easier)

-OR-

Derelict Prison

• you’ll meet Ghost: answer his question correctly („Coronzon is.“) → TIP 10
  
• fight the Gells (harder)

en route between Water Menhir and Opacum

• if/when you meet Nakoma/Atraakh in the forest, tell him "I can't help you!"

Opacum

• talk to Coronzon, ask him if he’s still mad at Treeg, then tell him to forgive him
  
• check out the menhir and read the inscription → INSCRIPTION 6
  
• you’ll get INSCRIPTION 7 after fighting Amma

Generally

• Be nice to people, they might share their strixes. You should not be running into resource problems with this group.
  
• You must make good/merciful choices (check the walkthrough for outcomes,) because Lo Pheng has to not be an emotionless killer by the time he reaches Opacum – otherwise he will kill Reet.
  
• You must keep morale at minimum 30%, more is better. Your party feels uneasy around you, so don’t talk to them unless you have to (noted below) – do stuff they find encouraging instead: be kind to randos and don’t avoid fights with Frisians. Win them epically instead.
  
• Exception to the last point: talk to your entire party after meeting Amma.
  
• Reet cannot die under any circumstances
  
• more fights = more strixes
  
• same as with Thorn: don’t rest/heal, it’s too expensive
  
• optional personally, Bolla is too much of a weak glass cannon for me, use two archers or more frontline instead

en route to Triple Menhir

• when given the choice, share your clan’s strixes with Reet to save her life

Triple Menhir

• choose the „Path of Mercy“ to continue

after meeting Khama

• you can now get My foe is my friend! See Alina's guide for details.

If you end up with a playthrough where Lo Pheng kills Reet in Opacum anyway (but morale of Pheng's party was high and Hopper solved the entire riddle) due to Thorn's influence, try reloading around the Water Menhir (before Thorn reaches Opacum) waste some time - needs to be after day 45 - and then head to Opacum. You will be intercepted by a Frisian patrol. Try to flee, which will result in an automatic TPK, also giving you the Stupid Death achievement earlier than planned (we'll use this trick a few times). Alternatively, let Thorn die in battle.

Reload your save game at the Water Menhir with Hopper/Blance. For the Theopany achievements, you need to not solve the riddle completely; as suggested in our trusty achievement guide, don't answer Bernard's or Ghost's (depending on which one you meet) questions so you don't get their tips and don't read the Menhir's inscription at Opacum (just don't go to the Menhir).

Choose Thorn to finish the game.

Reload your save at Opacum, before the final decision.
Choose Pheng.

We're doing Armageddon, Fortune and Defeater of Death during this run because (1) they use cards, contradicting Tactical Genius if you don't reload immediately and (2) playthroughs in which Hopper accepts the curse are going to be a lot easier, making them better for doing specific battle stuff.



General Notes

• you can use Gleda in fights now! And cards! (and you should do both)
  
• other than that, follow the same roadmap as for playthrough 1
  
• visit merchants and keep an eye out for Sacrifice card shards
  
• once you have the card available, you can go for Fortune (unless one of your strong units has 3 wounds, which shouldn’t be the case anyway) – Alina’s guide has a detailed description of who to level and an example fight (skip to Fortune chapter).

• follow the same roadmap as for playthrough 1
  
• again: you have to solve the entire riddle

General Notes

• you won’t make it to Opacum, so we won’t have to worry about Pheng killing Gleda. Yay!

Gellian Tunnel

• at the entrance is a good chance to do Armageddon.
  
• at the exit, nicely enough, you can snatch up Defeater of Death.
  
• check the guide for details on both of these if you need 'em

en route to Triple Menhir

• when given the choice, don’t share your strixes with Reet – she’ll die

That’s it, this party is doomed now, just get to the Roaming Menhir as quickly as possible (or finish any immediate achievements you haven’t yet) and watch their demise.

Opacum

• choose Thorn

Reload after finding Mact. Waste some time - camp & heal, take the longest paths - and let Thorn's party die by trying to flee from the Frisian patrol near Opacum.

• has to be after day 45
  
• having the whole party die is the quickest and safest option to kill both Thorn and Gleda; if others die in battles before then, it doesn't matter

Basically the same as 1.1, reload at the Water Menhir with Hopper, don't solve the riddle completely as described.
Let Thorn's party die by fleeing the patrol, then finish the game.

Generel Notes
in order to kill everyone, you first need to recruit everyone (not enough to just kill whoever you have)
you might think just running through and killing everyone with the patrol works. It doesn't. Mact has to be killed by Adna for some reason, Foshta & Andra leave at the White Tower if you don't let them die before and Fisk's death at the Arch Menhir doesn't count.

en route to Dynford Menhir

• meet Vai, let him join - take a quick route there!

Arch Menhir

• help/save Fisk

Ursus

• go to jail
  
• have to meet Foshta & Andra
  
• let Foshta & Andra die ASAP (before White Tower)

Friga

• fight the Gells, save Hederlig

White Tower

• save Mact in time (he has to keep his hand)

Water Menhir

• be rude to Adna
  
• when Adna asks about the Umbra, tell her "For my sake" → she kills Mact

en route to Opacum

• don't reach Opacum before day 45
  
• when encountering the Frisian patrol, try to flee
  → everyone dies

Forest meeting with Nakoma (after White Tower)

• reply with "I can't help you Atraakh!"

Wodan

• save Reet

Gellian Tunnel to Triple Menhir

• save Ruor
  
• take route: Black Temple → Gellian Altar → Triple Menhir
  
• engage in all fights & let Reet get 3 wounds
  
• save Alus
  
• share Pheng's strixes with Reet

Triple Menhir

• save Khama & let him join

Reet must fall in battle before the Roaming Menhir!
→ everyone else dies

General Notes

• rob, kill and fight at every chance you get, do not ignore encounters (unless stated otherwise)
  
• gather all possible strixes, including graves & altars
  
• be rude and mean to everyone, including your children, keeping morale as low as possible
  
• keep them as long as you want, but you have to let Hode and Flitt die in battle before Opacum; you can also let Flitt die by siding with Krieger against him in Albius
  
• keep Sopp alive, he has to make it to Opacum

Albius

• don't buy anything from Padagang
  
• talk to Rask instead
  
• side with Krieger over Flitt

Watchtower

• tell Krieger to kill Stein upon leaving
  
• head to Dead Spring to avoid meeting Vai

Dynford Menhir

• accept Ramlin's strixes; never return them, given the opportunity
  
• lose Fisk to wounds before reaching the Arch Menhir

Arch Menhir

• don't tell Vegen about the use of strixes and don't share yours
  
• don't help Chila, you might just want to avoid talking to her

en route to Marsh Watch

• kill Helm
  
• when you meet Skoor, tell him that Helm was a betrayer

Marsh Watch

• ignore the bandits (avoid saving Rask)

Ursus

• at the gate, tell Sheck "What use of your service...", then "We're going to Unda..."
  
• if you want to do any shopping, do it now, you're about to leave
  
• don't visit Unda, go to Teek's tavern instead
  
• fight Brann, then leave the city

Friga

• don't fight the Gells (avoids saving Hederlig)

White Tower

• you have to get there in time to save Mact & Mact's hand

Water Menhir

• be rude to Adna, tell her "For my sake"

General Notes

• don't intervene in fights, never help anyone and choose evil actions whenever possible (refer to walkthrough linked in the intro section)

Wodan

• kill Reet (you psychopath)

Gellian Tunnel

• do not help Ruor (watch him die, then fight)

en route to Triple Menhir

• don't ever help Alus, don't intervene when Shannon is feeling edgy

Opacum

• side with Frisia*

*only works in this run because you killed Reet at Wodan, avoiding the Roaming Menhir and the resulting TPK there

General Notes

• never help Ake when given the option
  
• always resist the curse, never fighting the Reaping
  
• avoid solving the riddle at every turn (no tips, no inscriptions), see below

Albius

• don't talk to Coronzon; if it can't be avoided, don't demand a favor/side with Treeg against him

Dead Spring

• don't ask the girl if she needs help - or just don't go to the Dead Spring

Arch Menhir

• just don't talk to Vegen about Thorn/Chila and don't visit the Menhir (or at least don't read the inscription)

en route to Ursus

• ignore all trees you encounter (don't investigate, you'll find an inscription)

Ursus

• do not go to Unda's house
  
• Sheck will give you a tip when you leave; I don't know if this can be avoided

Scarlet Tower

• don't help Atori

Marsh Watch

• avoid talking to Rask/Chirlan (too much), or he'll give you a tip

Warden's Mansion

• don't help the warden

Friga

• do not enter Friga (avoiding helping Lon)

Forest meeting with Nakoma

• tell Nakoma/Atraakh: "I can't help you"

Water Menhir

• don't read the inscription, whether Adna tells you to or not ;)

en route to Opacum

• when meeting Bernard or Ghost, do not answer their questions

Opacum

• don't ask Coronzon "Are you still mad at Treeg", don't call in your favor if you set it up in Albius
  
• do not read the Menhir's inscription

you'll have to reload at the choice point to pick at least Lo Pheng (for Oath-keeper) and Hopper

Lo Pheng: choose Pelko Soturi
Hopper: kill Lo Pheng, defeat Atraakh
Thorn: kill Brann, Amra, Hopper, defeat Atraakh

Okay well.


These are very straight-forward, there's nothing to figure out here except that the game is now brain-meltingly hard.
The same user made another detailed guide for this:
https://steamproxy.com/sharedfiles/filedetails/?id=2035443050
Good luck!