Two-Weapon Flurry

Epic Tier
Prerequisite: 21st level, Dex 19, Two-Weapon Fighting
Benefit: While holding a melee weapon in each hand, if you make a successful opportunity attack with your primary weapon, you can take a free action to make a melee basic attack with your off-hand weapon against the same target (but with a -5 penalty to the attack roll).

Published in Player's Handbook, page(s) 207.