Eternal Thirst is a common Enchantment — Aura card from Magic 2015 (M15), collector number 95. Current market price: $0.14 (foil $1.94), TCGplayer market prices via Scryfall.
Enchant creature Enchanted creature has lifelink and "Whenever a creature an opponent controls dies, put a +1/+1 counter on this creature." (Damage dealt by a creature with lifelink also causes its controller to gain that much life.)