Dead Dealer's Dungeon Curse

Before you take this feat you must first lose to Biaxti and his accomplices in the Dead Dealer's Dungeon. 

Branded on the lower back with a dice embossed on a tombstone as a reminder of a failed attempt in a dungeon. As a result you now have the ability to Wager on an attack roll

Once per long rest, on an attack roll subtract 1d4 from your attack roll. if the attack hits add an additional 3d6 damage. If the attack misses you take 1d6 Psychic Damage.