Ragnarok is a special ability present in Kingdom Hearts and Kingdom Hearts II: Final Mix+. In Kingdom Hearts, it is learnt after Sora's second encounter with Riku, or after Riku is under the possession of Xehanort's Heartless, to be precise. However, it unaccessable once it is learnt because Sora becomes a Heartless immediately after the battle. Sora can only access this ability once his Heart has been restored by Kairi and the susequent series of events have transpired.
Appearances
Kingdom Hearts
Ragnarok
AP cost: 4
MP Cost: 3
Description: A flurry of air combos which ends with a laser burst. Deals Light-elemental based magic damage.
Effect: When close to an airborne enemy, Sora will automatically jump up to the enemy and deliver a series of mid-air slashes. Unlike Sonic Blade or Ars Arcanum, it requires no timing to execute the finisher, Burst; however, it should be noted, the longer Sora charges for the Burst attack on air, the heavier the damage inflicts. Aside from that, the lasers fired also have homing capabilities, which target and destroy the nearest foes in sight. Sora can move freely while the lasers home by themselves.
Kingdom Hearts II: Final Mix+
Ragnarok makes a comeback as one of four of Sora's former abilities to apear as one of Sora's Limits while Limit Form is equipped. Activating Ragnarok will trigger a Reaction Command, Burst, and just as in the first game, 
has to be hit at the right time for more projectiles to be fired. Hitting too early will cause fewer lasers to be fired.