Diseased Vermin

Whenever this creature deals combat damage to a player, put an infection counter on it. At the beginning of your upkeep, this creature deals X damage to target opponent previously dealt damage by it, where X is the number of infection counters on it.

From Magic: The Gathering.

Open the interactive version on https://mythscribe.cloud/app/tcg/cards/cms6o8ghv5dvvpb1hjprdnq7e.