Tainted Observer is a uncommon Creature — Phyrexian Bird card from Phyrexia: All Will Be One (ONE), collector number 217. Current market price: $0.27 (foil $0.31), TCGplayer market prices via Scryfall.
Flying Toxic 1 (Players dealt combat damage by this creature also get a poison counter.) Whenever another creature you control enters, you may pay {2}. If you do, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)