This page of the Baldur's Gate 3 guide includes the walkthrough of the Hunt the Devil quest. This walkthrough will help you complete the quest successfully.
- Find the devil;
- kill Karlach (optional);
- kill Anders (optional).
There is a small hut (m4;3) near the cave where you fought the gnolls (m4;3) on The Risen Road. a group of Paladins can be found there. Talk to their leader, Anders, and you'll know they're hunting the devil. Offer your help to start the quest.
The wanted - Karlah - is near the paladin camp (M4;2) by the river. When you get close, the conversation starts. The Tiefling will tell you the opposite - she has escaped from hell, and the group you met wants to capture her and bring her back. You can solve this task in two ways.
Kill Anders and the rest of the Paladins.
It's a more profitable option - you'll get more XP and better equipment. The fight will be moderately difficult. Pay special attention to Anders. He can deal severe damage and quickly knock down weaker teammates.
After the battle, collect the loot from the defeated and return to Karlach. It turns out that she is in fact the devil, but the Paladins are also messengers of hell. After this conversation, the quest is over.
Once you reach the river and talk to Karlach, you can just fight. This skirmish will be less demanding than the one against Paladins and shouldn't cause you any problems.
After killing Karlach, jump to where the Tiefling's corpse lies. You have to tear her head off to have proof for the Paladins. You can also examine her corpse, which requires passing an Arcana test.
Finally, return to the leader of the Paladins and hand him over the head. He will offer you a rare sword as an expression of his gratitude. After this conversation, the quest will be completed.