Swarm (G2)
From MediaWiki
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.
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.
Some stories (and extra-cannonical comments from the Beast Wars creators) suggested the Swarm eventually evolved into the alien race known as the Vok.
