World Seed
If you’re rolling Losomn world seeds, you will know you have the right seed if you start in Beatific Palace in front of a 2-sided door.
Faelin Mural Piece
While exploring Losomn with the Imposter questline, you will come across the Faelin Mural Piece near the start of the world.
Magic Quill
Look around the palace for a jester sitting on a pile of bones. He will give you the Magic Quill which can complete the drawn doors on walls in the area. Go through the doors, looking for ones that lead you into parts of the town at night. Verify that you are in the right location by interacting with the checkpoint and checking the fast travel menu to ensure the quest marker is on your current location.
Faerin Mural Piece
Continue exploring until you reach the area boss and exit through another wall door. This will place you near the second jester. This one will shuffle 5 cards and show you the one he wants you to track. Keep your eye on the card as he throws it against the wall creating a door that leads to the Faerin Mural Piece.
Finding the Imposter
Now, return to the 2-sided door at the start of the world and insert both mural pieces. Then, pull the switch to change it back to daytime. Once it is day (Beatific Palace not the Malefic Palace), enter the gateway and confront the boss. Make sure you are talking with Faelin during the day and challenge him.
Faelin Boss Fight
Faelin’s weak point is his head, so aim your shots accordingly. In the first phase, Faelin with lunge toward you and do a 2-3 hit combo. Be prepared to dodge out of the way multiple times. When he summons 1-2 floating orbs, take them out to ensure they don’t interrupt your attacks. If he summons a row of orbs, move far away from them, they have a short range. When Faelin reaches 50% health, he will transition to the second phase. You cannot damage him between phases.
During phase 2, Faelin will add large swords periodically dropping from above. Keep an eye on them and continue burning down his health bar. When Faelin reaches 25%, he will transition to the final phase.
In phase 3, Faelin will also add many small swords dropping from above and it will be difficult to find a location where you’re not getting hit frequently. He will also dash through you and attack from behind. If you are standing near a wall/corner, he will dash up against the wall, making it easier to shoot him at close range. Once Faelin falls, the Traitor achievement will unlock.