Sigil of Valor is a uncommon Artifact — Equipment card from The Brothers' War Retro Artifacts (BRR), collector number 53. Current market price: $0.28 (foil $0.32), 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.)