The Mirrorworld is in abrupt danger...

Among the different multiverses that exist, all of them converge at once, and each and every big bad that was defeated by Kirby are releasing all of their energy at once, causing them all to show up in one place at the exact same time...

Well, there's a MUGEN fight for this, before the sprite animation comes out, so here's the MUGEN battle:

https://youtu.be/8qrcX87RzDI