Self-milling swarm

Whenever a player casts a creature spell, each player returns all cards with the same name as that spell from their graveyard to the battlefield.

Whenever this creature attacks, choose a card name. If defending player has no cards with the chosen name in their hand, they may reveal their hand. If they don't reveal their hand, this creature can't be blocked this turn.

This creature gets +1/+1 for each other creature on the battlefield named Relentless Rats. A deck can have any number of cards named Relentless Rats.