Beware The Guardian of Rathmana and The Protector of Rathmana, twin level 55 Epic(x4) ^^^ mobs guarding one of the 4 exit passages. These will vanish after you have killed the other 3 named bosses in the starting area, one at the end of each of the other 3 corridors. They are Nagini, Sirn' Vehn, and Hira' Vehn. Each is a 51^^^, and a couple of them have some bodyguards. There are quite a lot of trash mobs to clear to get to each one.
Continuing then past where the epics were, you'll find Ortallius, a white NPC, surrounded by 6 Orillian Prelates and a huge statue in a shallow pond. The prelates are 50^^^ Heroics, but unlinked, and easy to pull individually. Apparently these cannot be either attacked until the first 3 bosses have died.
When all the prelates have been killed, Ortallius becomes attackable, he's a 51^^^, alone. Killing him spawns the final boss in front of the statue, Rathmana Allin, a 52^^^. She is surrounded by several Orillian Corpses, 51^^. But those are stationary, unlinked, and most are far enough away from her that they shouldn't be a problem. If you try killing them, they will respawn very rapidly (maybe in a minute or so) in other parts of the pool, so this is rather futile.