Acrobatic Backstab( Complete Scoundrel, p. 84)
[Movement, Skill Trick]
You dart past your opponent's attacks, ending up perfectly positioned for a devastating counterattack.
Tumble 12 ranks,
If you succeed on a Tumble check to move through an enemy's space, you can treat that enemy as flat-footed against the next melee attack you make against it on your current turn.
Your enemy must be standing on the ground or floor in order for you to use this trick.