Gloom Ripper is a rare Creature — Elf Assassin card from Lorwyn Eclipsed (ECL), collector number 312. Current market price: $0.42 (foil $0.63), TCGplayer market prices via Scryfall.
When this creature enters, target creature you control gets +X/+0 until end of turn and up to one target creature an opponent controls gets -0/-X until end of turn, where X is the number of Elves you control plus the number of Elf cards in your graveyard.