Bowser Jr.'s Lava Reactor
From Lumapedia
Bowser Jr.'s Lava Reactor is the fifth and final accessible galaxy in the Engine Room.
This galaxy is a large lava planet that is covered in sinking platforms. Bowser Jr. uses King Kaliente to fight Mario/Luigi at the end. Defeating the boss drops a Grand Star that unlocks the Gate and the Garden.
List of Stars
Routing Info
Bowser Jr.'s Lava Reactor is accessible after the player has collected 45 stars.