That's just the boss rush--thirty bosses from throughout the game, broken up into groups of five that must be defeated in rapid succession. It's separate from the dungeon proper, only becoming accessible once you beat Enuo and get the Necromancer job. The sole reward for surviving it is another useless item in the tradition of the Omega Medal and Dragon Crest, called Proof of Crushing Evil.

First GroupWing Raptor, Garula, Siren, Magissa & Faltzer, Gilgame
Second GroupLiquid Flame, Sol Cannon, Purobourous, Minotaurus, Triton/Nergeid/Phobos
Third GroupBiblos, Tyrasaurus, Flying Dragon Weed, Archaeo Demon, Apanda
Fourth GroupChimera Brain, Adamantoise, Garkimasera, Calofisteri, Twin Tania
Fifth GroupHe Who Knows Everything, Stalker, Sand Worm, Atomos, Halicarnassus
Sixth GroupCrystals, Melusine, Catastrophe, Apocalyopse, Necrophobia