|Fast Facts on Phantom King & Klepto Mobile|
The Phantom King and the Klepto Mobile are the true final bosses of Sonic Mania. The player confronts both of them in Egg Reverie Zone, but only by completing the game as Sonic with all 7 Chaos Emeralds.
The Heavy King, now transformed into the Phantom King, will try to take the Phantom Ruby for himself and attack Sonic, only to be stopped by Dr. Eggman in the Klepto Mobile, who also wants the Ruby. The two wrestle for it, and ultimately take turns fighting over the Ruby to defeat a common enemy: Super Sonic.
For this fight, the player must fight two bosses at once, though only one at a time. The Phantom King will attack first and chase the player while surrounding himself with energy orbs that serve as a shield against Super Sonic. He will then attack by firing the orbs one after another at Super Sonic, hurting him and making him drop some of his rings. Once he has fired all of his energy orbs, he generates more to shoot at Super Sonic. The player must take great care and move in for the hit just before the Phantom King regenerates more energy orbs. Once the Phantom King is hit, he will drop the Phantom Ruby, which then ends up in Eggman's hands.
With the Ruby in his possession, Eggman flies in to attack Super Sonic. When the golden hedgehog flies near, Eggman will rapidly swing his arms to repel him, so the player must wait for him to stop swinging his arms before striking him and making him drop the Ruby for the Phantom King to pick up. If the player keeps their distance from Eggman, he will use a different attack: He will harness the Ruby's power to encase himself in a strange barrier, then charge into Super Sonic, knocking him away; however this attack does not make him lose any rings.
Both bosses take 8 hits each to defeat, leading to a total of 16 hits. Although it is possible to do otherwise, canonically the Phantom King is defeated first, with Eggman defeated shortly afterwards.