Necrobite is a common Instant card from Avacyn Restored (AVR), collector number 115. Current market price: $0.08 (foil $0.49), TCGplayer market prices via Scryfall.
Target creature gains deathtouch until end of turn. Regenerate it. (The next time that creature would be destroyed this turn, instead tap it, remove it from combat, and heal all damage on it.)