Crescendo of War is a rare Enchantment card from Commander 2011 (CMD), collector number 12. Current market price: $2.27, TCGplayer market prices via Scryfall.
At the beginning of each upkeep, put a strife counter on this enchantment. Attacking creatures get +1/+0 for each strife counter on this enchantment. Blocking creatures you control get +1/+0 for each strife counter on this enchantment.