Tabuu is the leader of the Subspace Army and the final boss of Subspace Emissary mode in Super Smash Bros. Brawl. He is a powerful, supernatural entity who resides in Subspace. He uses the Subspace Bombs to send entire locations to his dimension where he absorbs their power to increase his own. His name is likely a pun on the English word "taboo", which is to be unacceptable or improper by society's terms. He is a cold hearted ruler and because of him R.O.B. is the last of his kind.
Role in the Subspace Emissary
Tabuu is actually the embodiment of Subspace. His plan was to cut World of Trophies into pieces and take them to Subspace for his world. However, he himself can not leave Subspace, which is why he created the Subspace Army to act in his stead. According to the Ancient Minister trophy, Tabuu took over the Isle of the Ancients, the home of the R.O.B.s and forced them to serve him under the threat of annihilation. Tabuu first discovered that Mr. Game & Watch had Shadow Bugs in him and captured him to harness the power, and thus the Subspace Army was born. Tabuu studied and took control of the master of the Smash Bros. world, Master Hand, who enlisted the help of Ganondorf, Bowser and Wario. These three would hunt down any fighters that would stand in the way of Tabuu's plans.
Later in the story, after the heroes destroy the Subspace Gunship and enter Subspace, Ganondorf betrays Bowser by using a Dark Cannon. Ganondorf then goes to greet Master Hand. Ganondorf realizes that Tabuu was actually controlling Master Hand and that he was not actually following Master Hand's orders but was being tricked by Tabuu. Enraged, he charges a futile attack at Tabuu who quickly turns him into a trophy. Ganondorf's body hits the puppeteered Master Hand, thus breaking him free of his Chains of Light. Master Hand also charges at Tabuu, but to no avail. Tabuu uses his Off Waves to turn everyone into a trophy as soon as they approach.
However, Luigi, Kirby, and Ness are later revived thanks to the Dedede Badge, along with King Dedede himself and he joins the team of Luigi and Ness, and they rescue everyone. Even the villains, such as Ganondorf, Bowser, and Wario, have been revived, and they team up with the heroes to take down Tabuu. Tabuu then creates a maze out of the worlds that have been consumed by Subspace and brings back the old enemies. He also creates dark clones of all the heroes to slow them down. Once the heroes have reached Tabuu, he will attempt to turn everyone back into trophies again. Before Tabuu can unleash his attack however, he is ambushed by Sonic the Hedgehog. Sonic then teams up with the heroes and villains to fight Tabuu in a huge battle. In the end, Tabuu is defeated. After his defeat, the Subspace Army is gone, lands are restored, and the Isle of the Ancients disappears afterward because of the effect of so many Subspace Bombs going off at once (commanded by Ganondorf so Tabuu could get a Subspace Gunship into the World) making it impossible for the Isle to escape Subspace.