Stensia is a common Plane — Innistrad card from Planechase 2012 Planes (OPC2), collector number 34. Current market price: $2.25, TCGplayer market prices via Scryfall.
Whenever a creature deals damage to one or more players for the first time each turn, put a +1/+1 counter on it. Whenever chaos ensues, each creature you control gains "{T}: This creature deals 1 damage to target player or planeswalker" until end of turn.