Eidolon of Countless Battles is a rare Enchantment Creature — Spirit card from Secrets of Strixhaven Commander (SOC), collector number 144. Current market price: $0.27, TCGplayer market prices via Scryfall.
Bestow {2}{W}{W} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached.) This creature and enchanted creature each get +1/+1 for each creature you control and +1/+1 for each Aura you control.