Haunted Plate Mail is a rare Artifact — Equipment card from The List (PLST), collector number M14-212. Current market price: $0.35, TCGplayer market prices via Scryfall.
Equipped creature gets +4/+4. {0}: Until end of turn, this permanent becomes a 4/4 Spirit artifact creature that's no longer an Equipment. Activate only if you control no creatures. Equip {4} ({4}: Attach to target creature you control. Equip only as a sorcery.)