After killing the remenant of Miragul the four chests in the room will become openable. Three of the chests will spawn groups of 4 a petulant force mobs. Those are level 48. Two of them have one arrow up and two are of standard strenght.
Myrgorth is a huge leve 51^^^ manticore. He will become attackable when stepping into the center of the room. Hell spawn three a dread servant to aid him in combat. Those are level 50 standard mobs. The named and the servants are seperate encounters.