Chainsaw is a rare Artifact — Equipment card from Duskmourn: House of Horror Promos (PDSK), collector number 128p. Current market price: $1.22 (foil $1.34), TCGplayer market prices via Scryfall.
When this Equipment enters, it deals 3 damage to up to one target creature. Whenever one or more creatures die, put a rev counter on this Equipment. Equipped creature gets +X/+0, where X is the number of rev counters on this Equipment. Equip {3}