Counterattack

This spell grants the target the ability to attack whenever they are attacked, regardless of whate else they do in a round. While the spell is in effect, if anything makes a melee attack against the character, the character instantly reacts with a melee attack of his own. The nature of the spell is such that even surprise attacks are subject to being countered. The number of attacks doesn't matter, and the spell doesn't protect against the damage dealt by them or other effects from them. Also, the other actions you may take while this spell is in effect are limited -- counterattacking takes at least one free or equipped hand, and interrupts any actions taking more than that, such as spellcasting.

Mechanically, this spell forces the character to make a melee attack whenever damaged in combat against the thing that damaged him. The attack happens automatically, and attacks in the direction the attack came from. If there is a creature there, the user of this spell may make an attack roll as a normal attack action to hit with whatever weapons they have in their hand at the time. These attacks do not count against the usual number of attacks allowed per round. If there isn't anything in the adjacent square (such as if the attack was a missile attack), you attack anyway, dropping whatever you were doing, though it is ineffectual.