Citadel Siege is a rare Enchantment card from Commander Anthology Volume II (CM2), collector number 21. Current market price: $0.35, TCGplayer market prices via Scryfall.
As this enchantment enters, choose Khans or Dragons. • Khans — At the beginning of combat on your turn, put two +1/+1 counters on target creature you control. • Dragons — At the beginning of combat on each opponent's turn, tap target creature that player controls.