You instantly attack when an enemy comes too close. You then quickly step away.

Encounter   ✦     Martial, Weapon
Immediate Interrupt      Melee weapon

Trigger: An enemy moves during its turn to a square adjacent to you

Target: The triggering enemy

Attack: Dexterity vs. AC

Hit: 2[W] + Dexterity modifier damage, and you shift 1 square.

Published in Martial Power 2, page(s) 39.