Revenge of the King is a new mode for Kirby Super Star Ultra, and can be unlocked by completing The Arena. At its cored, this is a more challenging remix of Spring Breeze, but this time the stages are harder and longer, the enemies are more difficult and more numerous, and there's tougher bosses looking to take Kirby down. The basic structure of each stage is similar, but with recolored sprites and added length. The Mid-bosses have been replaced with Mid-bosses from other Kirby games, and there are brand-new enemies scattered around too. Additionally, the final stage has diaologue coming from King Dedede and his minions, similar to Meta Knight and his comrades in Revenge of Meta Knight.
King Dedede is seeking vengeance for his defeat in Spring Breeze. It is unknown why Kirby has accepted the challenge and ventured to the King's castle--perhaps the pink lump is nearly acting out of honor. On a side note, the anonymous Waddle Dee with way too much HP in The Arena has been given a backstory in this new mode.
Completing this mode unlocks Meta Knightmare Ultra.