Sigil of Valor is a uncommon Artifact — Equipment card from The List (PLST), collector number ORI-239. Current market price: $0.16, TCGplayer market prices via Scryfall.
Whenever equipped creature attacks alone, it gets +1/+1 until end of turn for each other creature you control. Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)