Norn's Choirmaster is a rare Creature — Phyrexian Angel card from Phyrexia: All Will Be One Commander (ONC), collector number 8. Current market price: $12.14, TCGplayer market prices via Scryfall.
Flying, first strike Whenever a commander you control enters or attacks, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)