Doubtless One is a uncommon Creature — Cleric Avatar card from Onslaught (ONS), collector number 27. Current market price: $0.21 (foil $3.35), TCGplayer market prices via Scryfall.
Doubtless One's power and toughness are each equal to the number of Clerics on the battlefield. Whenever this creature deals damage, you gain that much life.