The're only 2 bosses in this instance.
Both bosses have an achievement.
The first one is called bronjahm<godfather of souls> or however you spell it. The're 2 phases. In the first phase he will randomly target a player and casts corrupt soul the player that gets the spell should run away from the party And then the dpses should slow and kill the corrupted soul (small purple ball) that comes out of the player who got the spell on him. in the second phase he wil cast soulstorm when that happens he stays in the middle and casts shadow bolts (or something similiar) to the tank. all party members need to stay in close range to him otherwise you will get a lot of damage from the ring thats around everyone. He will also cast fear at random players from time to time the player should (if possible) break the fear or someone else needs to do it. If noone breaks the fear the healer should focus on healing the target. The achivement is to kill him when there are 3 corrupted soul fragments alive.
The second boss is the devourer of souls. He has a few things you should know. He casts mirrored soul sometimes when that happens everyone should stop dps'ing. He also casts unleash souls which make a lot of souls appear who cant be killed/taunted and will disapear after time the healer has the hardest job here. he also casts wailing souls then everyone including the tank should get behind the boss otherwise you will die from massive damage. For the achievement you need to kill him without letting him cast phantom blast so it needs to be interupted.