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.
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.