When the callback is executed, the following parameters are passed to it:
 - (string) name of the character
 - (string) name of the attacker, or nil if there was no attacker
 - (float) hit points after receiving the damage, as a percentage of maximum hit points
 - (boolean, only for characters and vehicles) true if the damage came from an explosion, or false if not
 - (boolean, only for characters) true if the damage came from a melee attack, or false if not
 - (boolean, only for characters) true if the damage causes the victim to vomit, or false if not