Tabuu is perhaps one of the biggest villains in the entire Lapisverse. Initially bound to his universe of Subspace, Tabuu had assistance from the multiverse such as Zim, Nazo, and others to take out the Fighters of Lapis so he could create a new universe in his name. Thwarted time and time again, he finally destroyed the entire Lapisverse with a black hole.
His failure can be associated with the fact he never sees any betrayals in the plan. He briefly ruled the Lapisverse for 3 years before the Uprising happened.
Fighters of Lapis
In Fighters of Lapis, he was behind the plot of the Space Crisis as the mastermind; his subspace bombs took out many locations and he used Nazo, Zim, and Darkseid to help him out. Nazo, who had been brainwashed into helping him turned on him as soon as Zim did. Taking out both, but was suddenly attacked by Sackboy who had come out of nowhere, he resorted to fusing with Darkseid to take out the Fighters of Lapis. He failed, and the worlds are restored but his defeated body comes across Missingno.
Fighters of Lapis 2
In Fighters of Lapis 2, he had corrupted Missingno to use his glitch powers to control the entire Lapisverse. He was defeated by the Fighters of Lapis and Ashura, who wanted to become a real character. Defeated once again, it is assumed that he came across other villains who would help in his third attempt.
Fighters of Lapis 3
In Fighters of Lapis 3, he teams up with the Aurum and several villains to take control of the Lapisverse. It ends up working and he controls the Lapisverse, taking care of spots that were either out of reach or uprising against him. He uses the Monolith to find a blank universe so he can create subspace (with a power known as Irregular) in after realizing that the uprisings won't stop until he becomes the new god. Finally disappointed with how the plan is quickly coming to shambles and not willing to face defeat anymore, he destroys the Lapisverse with a black hole that nobody is quick enough to stop. He goes in as a trophy, possibly never to be seen again...
Fighters of Lapis 4
Although he does not appear, he is responsible for restoring the memories of the Fighters of Lapis through Namine.
Fighters of Lapis 5
SPOILERS BELOW FOR FIGHTERS OF LAPIS 5
Tabuu returns in the last part of the game. He and his past self are the two final bosses, and once he has been defeated he goes disappears and the Fighters of Lapis return to the real world. In a post credits scene he is shown heading towards the golden castle of the Brotherhood.
Fighters of Lapis 6
Tabuu makes his big return with the Brotherhood, joining with Taizo and Bosslady to defeat the Fighters of Lapis.
Virility Melee (SPOILERS)
It is revealed that both Bill Cipher and Hades are working for a man named Tabuu. Tabuu was severly injured after the events of the Subspace Emissary, rendering him useless. After defeating both Bill and Hades, Tabuu goes insane, causing him to rip open his chest, revealing the true final boss: The Swarm.