undying swarm

indestructible

at the beginning of your upkeep put a wave counter on undying swarm

at the beginning of your upkeep if you don't
have a rat wave creature token with blitz{3}{b} and this creature's power toughness is equal to the amount of rats you controll then create x black rat wave creature token where x is the number of wave counters on this permanent

if you controll no rats put this card in the graveyard