I can't greet people with an intro, sadly, due to the character limit. The boss'd be an Ascian Lord.
Phase 1; Boss has about 30k HP. Only an incredibly weak autoattack. Once slain, two chests would pop up like the boss was fully slain. The U.I. for completing the dungeon (Commend Menu, etc.) wouldn't, though. Anyone who walks near those chests would die instantly. HP refreshes if slain.
Phase 2; Boss has about 50k HP. A slightly strong autoattack with Fire and Blizzard to top it off, and one additional ability, named Dark, that inflicts unaspected damage and a Healing Power Down + DoT effect. Dark is usually aimed at the healer. He'd also be able to summon 15 adds, 10 of which are incredibly tough but have nearly no damage, 5 of which have a moderate amount of HP and deal lots of damage. HP refreshes if slain.
More below. Character Limit.