Tempus is a ghost from an alternate future where he has taken over the world.
Tempus was a Class 6 who opened the Containment Unit on July 30, 1997 and led the Great Spirit Uprising. Despite the heroics of Kylie Griffin, Eduardo Rivera, and Dr. Roland Jackson in the Great Ghost War, Tempus remained ruler of Earth, led his ghost army, and enslaved all of humanity. He then destroyed most historical records. The last remaining band of freedom fighters rigged a Ghost Bomb and Proton Cannon from what notes, journals, and other records they could salvage. One freedom fighter named Isaac thought he destroyed Tempus with the Ghost Bomb.
Instead, Tempus' essence was split in two and a Time Slip was created. Kylie Griffin and Isaac swapped places in the time stream. The Ghostbusters attempted to trap the past Tempus but it escaped the Ghost Trap and fled the Firehouse in confusion. He wandered to Times Square, where his future self communicated his mission to him: release all the spirits from the Containment Unit and fulfill the Great Spirit Uprising.
In the future, a prophecy foretold the return of Kylie Griffin and she would deliver humanity from the tyranny of Tempus. The Ghostbusters attempted to rewrite the future and send a Proton Pack to Kylie. Both Tempus' tried to prevent this but Kylie was able to secure one thanks to Garrett Miller's quick thinking. She followed instructions left in her diary to confine Tempus as the site of the Firehouse. Once the others did the same with the past Tempus, the Time Slip reopened and Kylie merged the two in the present/past. The Ghostbusters then successfully trapped him and the Time Slip closed, restoring all to their proper place. Tempus was then put into the Containment Unit, thus it was never destroyed and the Great Spirit Uprising never happened.
Powers and Abilities
Like a standard ghost, he can fly and go through walls but Tempus also has elemental powers, including the ability to generate and throw fireballs and summon a gust of wind. His most unique ability is to co-exist simultaneously in two different eras and sense disturbances in the time stream. However, oddly, his memories don't seem to follow his past half for much, since, for example, he does remember he enslaved humanity, but not how he started the Uprising without his future half telling him.