Unkillable Swarm

"Even when slain, a Host might not die entirely. Its body breaks down into a flood of dead animals of ordinary size — if a pack of werewolves kills a Rat Host, the only evidence of its passing is a pile of dead rats. And yet, one animal among the swarm inevitably remains alive, with a living shard within it. If the werewolves do not quickly locate and kill the shard-bearer before it can escape, the Host will eventually return — and likely remember who did it harm."

How could I emulate this ability? It sounds like Unkillable 1, where -10xHP would be total destruction of original swarm.

Unkillable Swarm