The Ultimate Lair is a bonus dungeon located in the Dead Dunes in Lightning Returns: Final Fantasy XIII. The dungeon can be accessed only on the "extra day" via a new Cactuar statue that appears outside the Ruins East Gate.
The Lair is full of Last Ones, and is imperative to visit if the player wishes to complete a certain side quest, as it is the only location during a regular playthrough where Meonekton Ω will spawn due to having to kill a prohibitively high number (65,535) of the regular monster otherwise. If Lightning already drove a particular species into extinction, those specimens do not appear.
Within the Ultimate Lair, the game clock doesn't stop, even during battle, and Chronostasis is disabled. Lightning must keep moving to reach the deepest level. After defeating the enemies on each level she gains access to warp devices that lead to lower floors. With a higher battle score, Lightning can jump several floors deeper.
Even in this dying world, plenty of people still believe in God. They believe that they are special, and that God loves them unconditionally. They believe he is looking out for them. Gods exist all right. But they've got their own agenda. They sure didn't make this world and put people in it because they love us. They just wanted to create new tools.
The Lair is an anomaly in time. With Lightning having saved so many souls in the twelve days after her awakening as the savior, a break in spacetime causes the lost thirteenth hours of the previous twelve days to be reinstated as another day.
Not much is known about the Lair itself. It shares architectural similarities with the Temple Ruins beneath the desert and the Temple of the Goddess. As Lightning moves further into the domain Hope tells her that the Chaos readings are off the charts, and Lightning speculates that God himself may be residing in this place outside of time.
Lightning theorizes that Bhunivelze may have been attempting to make a replacement for humanity in this place. Because humans are made from Etro's blood and Chaos, they are innately unclean and impure to Bhunivelze. Lightning is led to believe Bhunivelze does not have the best intentions of humanity at heart, and that like the fal'Cie, sees men as tools to be used and discarded. In this place, the God of Light was seeking to create a more powerful species, tools to be used that would bend to his will and were not weak like humanity. To this end, Bhunivelze's ultimate creation is Ereshkigal.
Lightning's defeat of Ereshkigal is testament to the strength of man, and Bhunivelze's underestimation of mankind, but Lightning wonders if everything she's been able to accomplish was only because of the power bestowed by the gods, and if she even has the right to be called human anymore.
Most floors contain Last Ones with most dropping accessories. The dungeon is a chain of chamber-to-chamber rooms connected with cactuar waystones. Depending on the battle rating on the current floor, the player will be able to skip as many as three floors. Last Ones that have already been felled by driving their species to extinction do not appear.