The Fate of the Universe

The Fate of the Universe is the only mission of Bowser's Galaxy Reactor, and is the final mission of Super Mario Galaxy, excluding The Star Festival. The mission's objective is to defeat Bowser for the final time and collect the last Grand Star.

Layout
Mario begins on a planet with stone walls and spotlights that determine the direction of gravity; the area a spotlight is shining on is gravitized, allowing Mario to walk on the walls. The player must cross the walls using the spotlight while avoiding Dry Bones and Fire Bars to reach a vortex that will bring the player to a lava filled planet with many sinking and moving platforms. A vortex at the end of the area will bring the player to an ice planet. As the player moves along, ice platforms will form in front of them, creating a path in the same manner as Space Junk. The vortex at the end of the ice planet will take the player to a sand planet where the player has to dodge tall Fire Bars on moving platforms to reach the vortex at the end. Next is a two-dimensional platforming challenge involving Space Junk. The player must Wall Jump and avoid Fire Bars while also navigating a gravity chamber at the end to reach the next vortex. The final vortex will take the player to a large lava-filled tube planet with many Thwomps, Bullet Bills, and Banzai Bills, as well as sinking and moving platforms. The Launch Star at the end will send the player to a stairway where a cutscene with Bowser and Bowser Jr. will begin. As Mario makes his way up the stairs, Bowser Jr. will send out meteors to destroy the stairs, similar to the stairway in Bowser's Star Reactor. Reaching Bowser will begin a dialogue in which he announces his plan to create a new universe in his own image, with Princess Peach by his side. He then pulls Mario and himself to a small grey planet, initiating the final boss battle.

Bowser Battle
Bowser will begin the fight using his signature ground pounding attack to send shockwaves at Mario. He will then curl up into a boulder and attempt to roll over Mario. Mario must spin attack Bowser's head to knock him out of the boulder and send him sliding around the planet. Once he is spun again, Bowser will release some Star Bits and the process to damage him must be repeated, though he will have to be spun twice after he is knocked out of the boulder.

In the second phase, Mario and Bowser are gravitated to another planet covered with spiky plants and rubbery bulbs. After breathing a few fireballs and ground pounding, Bowser will curl into his shell and roll after Mario. He must be hit with a rubbery bulb to stun him and send him sliding around. When he is spun again, the process must be repeated. Alternatively, if Bowser is standing next to a rubbery bulb during his initial attacks, Mario can stun him without having to deal with the shell attack. As before, Bowser must be spun twice after being stunned to move on to the final phase.

The third phase is similar to the previous battles with Bowser. Bowser will attack by sending out several shockwaves, firing fireballs, or by curling up into his shell and attempt to roll over the player. After the shell attack, Bowser will attempt to ground pound Mario. If Mario is standing on a blue orb, Bowser will shatter it upon landing, burning his tail, and will begin running around the planet. Spinning Bowser will send him spinning around the planet, and spinning into him again will damage him. Repeating the process and damaging Bowser a second time will defeat him, and reward the player with the final Grand Star.