You are the Queen/King of the Shadow kingdom, and you have been called by Princess Peach to discuss combining your Kingdom’s. And as you two walk through the halls you turn your head and see how there is a room with two Toad guards outside the door frame and a bird cage in the room with a…Green shell?…in it, you ask Peach about it and she responds

“Oh…that is..Bowser…we defeated him a month ago and…we’ve had him there for the time being…”

she says as she looks at the room in disgust