To answer your question the comic is mostly the telling on how. Dark-Kahn was fused with. Darkseid & Shao Kahn. There's also cameo appearance of Johnny Cage, Goro, Aquaman & Robin. Also John Vogel wrote the script to this book. Hope all this helps...

MK Secrets has a few pages on this. Anything else we should know of?

Thanks for the explination Samus, just one question, does it explain why only certain people are affected by the rage?

I don't believe it does.

The Rage is sparatic. They dont give a reason that it affects one character at a time. But the reason that Aquaman etc are not in the game is because page 7 says that "many beings vanish from existence". So Robin Aquaman Johnny cage and Goro vanish and of course we can assume that many more characters on both sides vanish.