Spiral Cut is an enemy ability used by Tidus in the Fiend Arena, which just deals moderate non-elemental physical damage to one party member. This ability is not used when Tidus is controlled by the AI in the party.
Spiral Cut I-IV were water-elemental physical attacks in the original free-to-play versions of the game. The abilities could be used by any party member as long as they had the Tidus signet equipped. The abilities dealt heavy water-elemental physical damage to a single enemy. Spiral Cut dealt more damage depending on the summon's rank and had increased MP costs for each version of the spell. The abilities were removed from the premium versions of the game.
Spiral Cut is one of Tidus's equipment Soul Breaks available from the sword Brotherhood (X) which at the expense of one Soul Gauge segment unleashes a powerful attack on all targets which deals physical damage equal to 480% power of a conventional attack. Bringing Tidus to battle and emerging victorious with Brotherhood (X) equipped, he can eventually master this Soul Break and be able to use it without the need to have it equipped.