Swordmage Feature Aegis of Assault

You create an arcane link between you and a foe, allowing you to instantly respond to its attacks with a counterassault.

At-Will        Arcane, Teleportation
Minor Action      Close burst 2

Target: One creature in the burst

Effect: You mark the target. The target remains marked until you use this power against another target. If you mark other creatures using other powers, the target is still marked. A creature can be subject to only one mark at a time. A new mark supersedes a mark that was already in place.
    If your marked target makes an attack that doesn’t include you as a target, it takes a –2 penalty to attack rolls. If that attack hits and the marked target is within 10 squares of you, you can use an immediate reaction to teleport to a square adjacent to the target and make a melee basic attack against it. If no unoccupied space exists adjacent to the target, you can’t use this immediate reaction.

Published in Forgotten Realms Player's Guide, page(s) 26, Player's Handbook 3, page(s) 152.