Rage Form is a Formchange in Kingdom Hearts III. It is similar to AntiForm and envelops Sora's Keyblade in darkness for quick and powerful attacks. It has a chance to activate when Sora reaches critical health, regardless of current Keyblade.
Kingdom Hearts III
During Sora's final fight against Xehanort in Scala ad Caelum, Xehanort uses the power of Kingdom Hearts to absorb Sora's light, forcing him into Rage Form. By attacking Xehanort, Sora is able to regain his light.
Rage Form turns Sora's clothes, skin, and Keyblade deep black, with glowing red eyes, an aura of darkness, and a fluorescent white swirling pattern on his pants.
When in Rage Form, Sora can use Riskcharge as a situation command, raising his Strength, at the cost of depleting half his remaining health. After using Riskcharge three times, Sora gains access to Rage Form's finish command.
Entering Rage Form replenishes Sora's HP so it is useful when in low health.