Sigil of Valor is a uncommon Artifact — Equipment card from The Brothers' War Retro Artifacts (BRR), collector number 116. Current market price: $0.26 (foil $0.27), 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.)