Flanker

When you and at least one allied creature to you are adjacent to an enemy and on opposite sides or corners of the enemy’s space, you are considered flanking the enemy, and when you are in this position, you and the allied creature both have advantage on melee attack rolls against that enemy. Additionally, when you make a critical hit against the flanked creature, you can add additional damage to the hit equal to your character level, this damage is the same type as used on the weapon that you made the critical hit with.