From the Super Mario Wiki
The Crystal King guarded the last Star Spirit, Kalmar, in the Crystal Palace. Crystal King's body is invisible and he can also make his cloak and crown invisible too. Crystal King can fly, summon Crystal Bits and shoot ice beams. When he gets desperate, he begins copying himself and recovering HP.
When Mario defeats him, Crystal King says, "Oh no! The last Star Spirit!! King Bowser, forgive meeeeeeeee!!". He then spins out of control and disappears while sparks are flying away from him.
Like the other characters, Crystal King appears in the parade at the end during the credits. Two Duplighosts are impersonating him, and the real Crystal King is carried away by his Crystal Bits.
The Crystal King is incredibly powerful. If not beaten quick enough, the Crystal King will recover 20 HP at least three times. He can attempt a freezing attack which freezes Mario and also deals six damage if not blocked. The Crystal King can split into multiple copies of himself in the air or on the ground. If Mario does not choose the correct copy on the next turn, the king will unleash a powerful mist attack which deals eight damage.
The Crystal King also makes an appearance in volme 27 of Super Mario-Kun. Before the fight, the Crystal King announces his presence using Ice Bolt by freezing Kooper. The Crystal King borrows his attacks from the original game such as hurling Crystal Bits and creating duplicates of himself. After an assortment of attacks, including Mario's using smelly clothing, Kooper uses a flaming shell to defeat the Crystal King.
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