In Crookback Hallow the bosses are, Bolfar the Engineer, Rotbelch the Chef, Lockjaw the Janitor/Jailer, Gristlecrunch the Shaman, and King Grotspew Crookback.
Bolfar the Engineer is the NPC that you have to fight to get Farcag Cellar Map, his placeholder is N/A (not one). Level and stars: Level 6 stars 2.
Rotbelch the Chef. His placeholders are Crookback waiter/ Crookback Porter. Level and stars: Level 15 stars 5.
Lockjaw the Jailer/Janitor. He is the NPC that you have to fight (and kill) to get Crookback jail key. His placeholders are Lockjaw Henchman. Level and stars: Level 18 stars 4.
Gristlecrunch the Shaman. His placeholder is Apprentice Shaman. Level and stars: Level 19 stars 4.
King Grotspew Crookback. He is the NPC that you have to fight (and kill) for the quest given by Dervla Mathanach in Farcag Castle Courtyard. His placeholder is Royal Steward. Level and stars: Level 20 stars 5.
DO NOT PAY ATENTION TO DUSTWITHER CATACOMBS I COULD NOT FIND THOSE BOSSES. I found Disciple Kiara on the furthest left room, surrounded by the level 50 boneslaves. She was on the dais.