Eldritch Erosion( Complete Scoundrel, p. 77)
Your attack can weaken a foe's resistance to magic.
Sneak Attack +4d6, Knowledge (arcana) 1 rank,
Your successful sneak attack reduces your target's spell resistance and power resistance by 5 (minimum 0) for 10 rounds. If you use this feat a second time on a target before the ten rounds have elapsed, the effect of the first expires. using this reduces your sneak attack damage by 4d6