From Pathfinder Howl of the Wild, which isn't enabled.

Moray Ambush

FEAT 17

Activity reaction

Trigger You are Hidden and a creature who hasn't detected you passes within 20 feet of you.

Like the moray eels athamaru communities raise and train, you know how to quickly strike from hiding. Stride or Swim directly toward the triggering creature and make a melee Strike against it. The target creature is Off-Guard to this attack.

Special If you're mounted on an eel, you can have the eel move instead of yourself.

Pathfinder Howl of the WildORC