(I don't know WHERE I got the idea!)
Combat Challenge <free action>
Trigger: Your most recent action was to make a melee attack.
In combat, it's dangerous to ignore a guardian. You mark the target you just attacked. The mark lasts until the end of your next turn. While a target is marked, it takes a -2 circumstance penalty to any attack roll that doesn't include you as a target. A creature can be subject to only one mark at a time. A new mark supersedes a mark that was already in place.