The Virus: Hegemonizing swarms is the term for the kind of Hive Mind that does this (most are perfectly benign non-coercive Fusions) – for as long as they exist, anyway, inasmuch as hegemonizing swarms, like Space Pirates, are acceptable targets for just about everybody.
The greatest and most long-lived of these examples is, of course, the Leviathan Consciousness, which still dominates the Ancal Drifts and Koric Expanse constellations – an aggressively hegemonizing perversion that came about from a runaway self-improvement cycle in software designed to optimize planetary networks by removing redundant processing. Guess how much redundant processing there is in sophont brains for it to optimize away?
And, of course, if you don’t mind getting on the short list of kill-on-sight galactic war criminals, you can do this with replicant nano- and neuroviruses, assuming you’re going up against relatively soft targets in artificial immune system terms. So far, no-one’s been all that eager to end up there in this particular way.