Niv-Mizzet, Guildpact is a rare Legendary Creature — Dragon Avatar card from Murders at Karlov Manor (MKM), collector number 319. Current market price: $0.36 (foil $1.50), TCGplayer market prices via Scryfall.
Flying, hexproof from multicolored Whenever Niv-Mizzet deals combat damage to a player, it deals X damage to any target, target player draws X cards, and you gain X life, where X is the number of different color pairs among permanents you control that are exactly two colors.