So what's the story behind World Breaker?

Not with that attitude.

Most don't actually tell us much, like this one for Nasus:

Worldbreaker Nasus returns to this realm to remind its inhabitants who truly owns their lives. Worship and devote your soul to him, or he'll take it by force.

Then there's one like this, which give a little bit more insight into what you could call quote unquote skin lore:

The nefarious sorcerer Karthus Lightsbane has vanished with the tome his party has been hunting for! Only unspeakable terrors await with it in his diabolical hands. Sejuani Dawnchaser, Jayce Brighthammer and Bard Bard have joined forces to track him down and cast away the tome for good. Will they succeed before it's too late?

{quoted}

Its a reference to the Hulk

This? http://static8.comicvine.com/uploads/scale_super/6/60791/2973309-3981427978-29732.jpg