Gandalf, Party Guest is a rare Legendary Creature — Avatar Wizard card from The Hobbit Commander (HOC), collector number 2. Current market price: $1.70 (foil $1.60), TCGplayer market prices via Scryfall.
At the beginning of combat on your turn, you may cast an instant or sorcery spell with mana value X or less from your hand without paying its mana cost, where X is twice the number of legendary Wizards you control.