Despana School( Drow of the Underdark, p. 56)
You have learned House Despana's fighting techniques, which involve teaming up against an enemy with summoned creatures.
As long as you and a monster you summoned threaten the same foe, you are considered to be flanking that foe even if you don't have an ally on the foe's opposite side.
In addition, once per round you can declare that a melee attack you make with a mace, heavy mace, or warmace against a flanked enemy is intended to open the enemy's defenses. You must declare this attempt before you roll your attack, and the attempt is wasted for the round if the attack misses. If the attack hits, your summoned creatures gain a +2 morale bonus on attack rolls and damage rolls against that enemy until the start of your next turn.