Karlach, Fury of Avernus is a mythic Legendary Creature — Tiefling Barbarian card from The List (PLST), collector number CLB-186.
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.)