Intersting concept, although the effect seems more like a premptive strike, not a counterstrike. I like the idea though.
As a true counterstrike mechanism, it could be attributed to a "eye-for-an-eye" version of justice, which may have a place in
... attacking before the enemy can attack seems slightly out of place though.
It seems more like a fire or darkness tactic.
Considering this creature is in an element that can buff it very easily, this could be an exceptionally powerful ability. If you have many of these in play, however, the effect may become a little too OP.