the most powerful weapon in Baldur's Gate 3

It's not a legendary 2 handed sword. It's not a reach weapon. Not the Devotee's mace, which requires a ONE TIME PER CHARACTER SPELL,

It's a one handed mace, and you can get an infinite supply.
The Deva Mace.

Currently, available in act 3, as soon as you get to the lower city.

Directly beside the Basalisk Gate, there is the Stormshore Tabernacle.

Inside, you can give five thousand gold to any statue and get a buff each long rest.

There is also a vendor with some nice items. But, downstairs is the real treasure.

There are many traps, so bringing a capable lockpicker slash trap disarmer is recommended. Once you make it to the room with 5 chests, you can begin.

Opening and looting any item from the chest (like your 5k gold) will dive you a curse. Castigated by Divinity. This will also annoy the shop owner, so don't let all your characters be cursed if you want to shop there later.

Using a scroll of remove curse, or spell will summon a Deva enemy. A not that tricky celestial you need to beat.

Once they're dead, they normally have nothing in their inventory, but, they are weilding the mace we want.

It is possible to do a long rest and come back, and the body of the Deva to be turned into a pouch, but who wants to wait.

Pick up the body with the right click option. You will need decent strength and empty room to hold the corpse.

Remove your weapon currently equipped.

Now, clicking on the empty MAIN HAND slot, you will see a small window with all the available weapons you can swap out. A special purple helmet icon will be on the mace the dead deva is actually using.

Click to swap in their OP mace into your hand, and discard the corpse.

You can repeat the curse by dropping any item into any of the 5 chests, then taking it back out.

The gods are stupid, and will recurse you. You can then repeat the uncurse, kill the new deva and get a new mace.

No mods. Works in most recent patch, patch 8.