From what I've seen Surprise Ending counts for a separate attack. I.e. it goes like this: opponent makes an attack attempt -> Surprise Ending triggers -> your gladiator makes a counter-attack attempt, which may use the properties of the opponent's attack (a special skill for example, if the opponent is using such at the given moment) -> the opponent receives a chance to block/avoid the counter-attack and if the dice fails him, he/she gets hit. The counter-attack doesn't necessarily aim at the same spot where the opponent's original attack is directed though.