Heartseeker is a rare Artifact — Equipment card from Darksteel (DST), collector number 124. Current market price: $2.52 (foil $9.05), TCGplayer market prices via Scryfall.
Equipped creature gets +2/+1 and has "{T}, Unattach Heartseeker: Destroy target creature." Equip {5} ({5}: Attach to target creature you control. Equip only as a sorcery.)