Mercurial Spelldancer is a rare Creature — Phyrexian Rogue card from Phyrexia: All Will Be One Promos (PONE), collector number 61p. Current market price: $0.66 (foil $1.07), TCGplayer market prices via Scryfall.
This creature can't be blocked. Whenever you cast a noncreature spell, put an oil counter on this creature. Whenever this creature deals combat damage to a player, you may remove two oil counters from it. If you do, when you next cast an instant or sorcery spell this turn, copy that spell. You may choose new targets for the copy.