Bhunivelze is the almighty god at the apex of the pantheon in the Final Fantasy XIII series. Though he is referenced in Final Fantasy XIII and XIII-2, he does not actually play a significant role in the story until Lightning Returns: Final Fantasy XIII, and like the other gods in the Fabula Nova Crystallis mythos, was initially never given a specific form. The symbol representing him resembles an eye and is referenced in the form Bhunivelze first uses to communicate with Lightning in Lightning Returns.
As the world is on the verge of collapse, with only thirteen days until its end, Bhunivelze awakens Lightning from her self-imposed crystal stasis and casts her in the role of humanity's savior. He tasks her with saving the souls of humans so that they may be reborn in the new world he intends to create as the current world ends. To help ensure her compliance, he also promises that he will resurrect her sister Serah in the new world if she succeeds in her mission. Secretly, Bhunivelze is also using the mission as a test. With the goddess Etro gone, a new goddess of death will be required to take her place, and with the divinity that he has imparted in Lightning, he wishes to see if she can grow strong enough fill that role.
His physical form is a metallic humanoid, with androgynous facial features, azure eyes, purple robes, and four pairs of angel-wings. An ornate crown-like adornment hovers between his horns. His name and the symbol of the Secutors can also be seen on the horns. Bhunivelze also wields a twin scythe with a demonic face representing Pulse at one end and an angelic face representing Lindzei at the other. Having a second face, Bhunivelze can also twist his body to assume a bird-like form with the scythe configured into talon-like projections.
Bhunivelze is a perfectionist with no compunction over disposing of those he uses and little tolerance to humans, whom he considers unclean because of the Chaos within them. As inferred in the Fabula Nova Crystallis mythology, he fears death and hates his mother and those resembling her. Lightning Returns: Final Fantasy XIII elaborates on this, showing that he also despises humanity for the darkest aspects of their souls; it is mostly this that motivates him to purge humans of their emotions, believing it would grant them eternal bliss. He also displays arrogance and cruelty, and even some basic emotions near the end, when Lightning succeeds in besting him in a one-on-one battle.
Despite seemingly being the most powerful deity in the Fabula Nova Crystallis mythos, able to rewrite a person's mind and purge them of emotions, Bhunivelze cannot see invisible entities, such as the Chaos or the souls of the dead. As such, despite his promises to Lightning, Bhunivelze could never truly fulfill his promise to resurrect Serah. At best, he could only create a replica; a being that at heart isn't the same as Lightning's true sister.
Bhunivelze is the final boss of Lightning Returns: Final Fantasy XIII. The battle against him comes in four stages, with his tactics and abilities changing with each phase. However, in a New Game Plus, the battle changes, and Bhunivelze has only two forms. The first is comprised of a combination of abilities from his first three standard forms, while the second is a powered up version of his normal final form.