At long last, the very core of the MegaDungeon - a prison for a mad lich. The lich was once a fire wizard who burned his way into this inner sanctum in pursuit of a safe place to hide his phylactery. He found it, but the Dungeongod tricked the lich by dimensionally anchoring him, waiting until he had wasted his gate and teleport spells, and then barred his escape with a creature immune to all of his fire powers.
Now the lich is trapped with his back and his phylactery against the wall. If the party manages to make into the lich's prison, they will have beaten the hydrax and now stand as the only things between the lich and freedom. Not only that, they are standing with yards of his phylactery. That's some pretty powerful motivation to bring the hammer down on them hard and fast.
No treasure listed here. As this is the ultimate level of the dungeon it should represent a definitive capstone. If the party needs more loot, you can stash a couple hundred thousand gold in the phylactery room along with an artifact or campaign specific magic item or two.