A conditionally cursed Robe of Blending (3 speeds). It was sized for a Small creature. Anyone who activated it would gain the stealth bonus but if they were larger than Small it would deal damage as it sliced them down to size. It would only attune to Medium (4d6 damage) and Large (4d10 damage) creatures.
For monster powers, I ran a demon-hunter game back in the early 2000s. Some demons could infect a pack of animals. But instead of using a mob template, each time a pack member was killed the others got a buff as the demonic energy concentrated in the remainder.
If you are aware of the Fractal Golem, think that but backwards. Start with 8 possed 4HD small demon-coyotes. Kill one and the survivors all heal 4d10. Kill a second one and they grow +1 size to Medium, heal another 4d10 and gain +2 attack/+5 damage, +2AC, +1 to all saves and 4HD. Repeat until you get to a Huge 16HD demon-coyote.
It turns one encounter into a whole plot arc. Especially as the demon summoner, if they exist, will eventually lose control as the possesed creatures' HD got too high.
This also works for Undead like vampires. Imagine a vampire making empowered servants (Renfields) rather than spawn. Doing so weakens the vampire while they live but as their servants die, they reclaim their power and get stronger.