Night Terrors: Alien Parasite (2005)

From publisher blurb:

Are your friends acting strangely? Are arachnids crawling out of your slain foes? Do you hear voices?

From Khan's Press comes a book that's going to get under your skin. Alien parasites are small insect-like aberrations capable of full character development. Though small enough to squish, they seek out other creatures to crawl inside and possess.

Alien Parasites:

• Possess other creatures, killing their minds and claiming the bodies.
• Unlock new evolutionary abilities as they gain Hit Dice.
• Progress with character levels.
• Are fully useable by both Players and Game Masters!