Type: Epic
Source: Epic Level Handbook
You can use countersong as a reaction to a sonic or language-dependent magical attack.
Prerequisite: Combat Reflexes, Perform 30 ranks, bardic music class feature
Benefit: You can begin a countersong at any time, even when it isn't your turn (much like a wizard who has readied a counterspell action), though you don't have to ready an action to do so. Thus, you could use the countersong as a reaction to a sonic or language-dependent magical attack. You can't use Reactive Countersong at the same time you are using another bardic music ability (though you could stop the other bardic music ability to begin Reactive Countersong if so desired).
Normal: Without this feat, you can only use countersong on your turn.