Fires when a request to cast a spell (on behalf of the player or a unit controlled by the player) is sent to the server
Signature:
("unitID", "spell", "rank", "target")
Arguments:
unitID- The unit that's casting. (string)spell- The name of the spell that's being casted. (string)rank- The rank of the spell that's being casted. (string)target- The name of the target of your spell. (string)