Swarm (G2)

Optimus Prime feeds the Matrix to the Swarm.

The Swarm, a cloudlike black mass, is a by-product of a form of Transformer reproduction called budding. It seeks out and destroys life, literally "eating" living beings, in a fairly horrific fashion. The Swarm is drawn specifically to Transformers, though it will eat other forms of life.

Fiction

Marvel Comics continuity

Optimus Prime fed both himself and the Matrix to the The Swarm, filling it with Primus's vision, and hence turning it into a force for creation rather than mindless destruction. In an attempt to make amends, it recreated Prime in a new body, before departing. A Rage in Heaven! This is, however, after The Swarm killed most, if not all of humanity. Great work, guys.

Trivia

• Extra-cannonical comments from the Beast Wars creators suggest that the Swarm eventually evolved into the alien race known as the Vok. The BotCon exclusive story, Primeval Dawn, states that the Vok were "tempered by the darkness of the Swarm," and "forged in the purity of the Matrix."

From Teletraan I: The Transformers Wiki, a Wikia wiki.