You're probably thinking of something else, but in core there's the
Hellwasp Swarm.
Inhabit (Ex)
A hellwasp swarm can enter the body of a helpless or dead creature by crawling into its mouth and other orifices. Inhabiting requires 1 minute, and the victim must be Small, Medium, or Large (although four swarms working together can inhabit a Huge creature). The swarm can abandon the body at any time, although doing this takes 1 full round. Any attack against the host deals half damage to the hellwasp swarm as well, although the swarm’s resistances and immunities may negate some or all of this damage.
If a hellwasp swarm inhabits a dead body, it can restore animation to the creature and control its movements, effectively transforming it into a zombie of the appropriate size for as long as the swarm remains inside. If a hellwasp swarm inhabits a living victim, it can neutralize the effects of its own poison and control the victim’s movement and actions as if using dominate monster on the victim. The hellwasps quickly consume a living victim, dealing 2d4 points of Constitution damage per hour they inhabit a body. A body reduced to Constitution 0 is dead.
A hellwasp-inhabited creature is relatively easy to spot, since its skin crawls with the forms of insects inside. The swarm is intelligent enough to attempt to hide beneath loose clothing or a large cloak to keep its presence from being detected. The swarm can attempt a Disguise check to conceal its inhabitation of a host, with a -4 penalty if currently inhabiting a Small host.
A remove disease or heal spell cast on an inhabited victim forces the hellwasp swarm to abandon its host.
