Swordmage Feature Aegis of Shielding

You create an arcane link between you and an enemy, allowing you to blunt its attacks against your allies.

At-Will        Arcane
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 interrupt to reduce the damage dealt by that attack to any one creature by an amount equal to 5 + your Constitution modifier.
At 11th level, reduce the damage dealt by 10 + your Constitution modifier. At 21st level, reduce the damage dealt by 15 + your Constitution modifier.

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