Well this is what i have of the story so far:


This is the World of Smash brothers. In this world trophies are brought to life and are forced to fight one another.

All they know is fighting, it is why they were created (by Master Hand most likely)

Being turned back into a trophy is basically death for the smashers. Here some might say that Mario is bringing Kirby back to life or into existance, but i think only Master Hand can do that, so i'm pretty sure Mario *kills* Kirby, and that this is one possible outcome from fighting.

However, there is another psooble outcome, to not fight and form an alliance. However this is breaking the Rules of the world.

When those rules are broken, the World will pay a terrible price. I'm guessing that Metaknight is some kind of enforcer of "The Rules", as well as these things:

I'm pretty sure that thing at the end is a minion for the main antagonist (Master Hand?). If you look at the picture you'll see two more of the purple "dark matter" forming together to the left and right of the thing, probably making more "things".
But all this is, is pure speculation.