Back in 1999 (yeah, I know I know), I had an idea for a villain that was a swarm of demonic creatures.  It was a single entity, had one attack, but consisted of a multitude of small demonic humanoids.  Not real original, but the creature was called The Wilding.  So far, just special effects.  Stretching powers to simulate the ability to be out of the way of attacks, or to get into areas smaller than one would expect.  But, the scary part was the power of the creature:

Poison/Venom - Swarm Infection
This is a terrifying power of the Swarm. It corrupts the individual, breaking it  up into multiple demons, which join the swarm. This occurs over multiple days, with accompanying swelling, bloating, tenderness at the wound, and fever. If the vicitm doesn't succeed in fighting off this infection, they will die. When death occurs, the body ruptures, with 1 demon per basic hit of the victim crawling out and running off.

This attack requires a special carrier attack in  Hand-to-hand. If successful, the victim must make an endurance save each day for a # of days equal to their
basic hits. The basic save is on d20 (tentatively modified by the Damage Modifier of the Swarm). Alternately, the victim only has a number of saves equal to their basic hits. If they fail all of them, they become a member of the swarm. They add a number of basic hits to the swarm equal to their own basic hits. This would only affect sentient beings.

(You can probably see that this was the original inspiration for my zombie bite idea.)