Karlach, Fury of Avernus is a mythic Legendary Creature — Tiefling Barbarian card from Battle for Baldur's Gate Promos (PCLB), collector number 186s. Current market price: — (foil $35.61), TCGplayer market prices via Scryfall.
Whenever you attack, if it's the first combat phase of the turn, untap all attacking creatures. They gain first strike until end of turn. After this phase, there is an additional combat phase. Choose a Background (You can have a Background as a second commander.)