Critical Hit
Critical Hit is a battle feature introduced in Kingdom Hearts. It is a finishing blow always present at the end of a combo and also during some magic abilities and possesses more damage power than the follow-up combination attacks. The magic ability Trinity Limit is rich in critical hits.
Furthermore, keychains and Sora's ability Critical Plus can help the chances of critical hits being present in a combination and also determine the strength of the critical hit. Examples of Keychains that encourage critical hits are Fairy Harp, Pumpkinhead, Olympia, Metal Chocobo, and especially Wishing Star. Depending on the Keychain's Passive Ability, the strength of the critical hit and its overall presence in the combo varies.
In Kingdom Hearts 358/2 Days, equipping a character with the Critical Ring, Lunar Strike or Striker Sun rings will increase Critical Hit Ratio, and equipping a weapon with Sight Units will also increase Critical Hit Ratio. The critcal hit is indicated when yellow lines appear from the attack and the attack has a change to its sound.
Importance
Critical Hits are important as they help with defeating bosses. Also if you are after the rarest item in the game, Mystery Mold, then you must defeat a Black Fungus right on a critical hit. It is best done using the Trinity Limit magic ability.
When in Atlantica, the easiest way to defeat a Sheltering Zone Heartless is to defeat it right on a critical hit. This stops it from exploding into more Sea Neons.