"A couple of years ago, in the Marvel and Capcom universe, a powerful being known as Onslaught was responsible to the near-death of entire civilizations.  To rid all reality of Onslaught's madness, I secretly banished Onslaught to a dimension known as The NetherRealm.  Now, after Shinnok's defeat at the hands of Earth's warriors.  Onslaught has escaped his confines of the NetherRealm to a dimension that surrounds all reality known to the Elder Gods as MUGEN.  The war is being fought once again.  Only this time, it's for all reality…" 

--These are the words of Raiden, god of thunder and lightning, and protector of the realm of Earth

Onslaught first appeared in the Marvel universe after merging with Professor Xavier, founder of the legendary X-Men.  Onslaught's power has been growing ever since the X-Men Vs Street Fighter crossover.  These crossovers were caused by Onslaught's ability to bend the MUGEN barriers and merge the Marvel and Capcom universes.  Onslaught fought the universes in the third crossover and lost.  Raiden sent the physical form of Onslaught to the NetherRealm, the same realm where Raiden banished Shinnok, a fallen Elder God.

Later, the Mortal Kombat tournament starts again (Mortal Kombat 4).  Shinnok managed to escape the NetherRealm and battle the Earth realm (of the "Mortal Kombat" dimension).  After Shinnok's failure to win the tournament and being sent back to the NetherRealm, Onslaught revealed himself to Shinnok and proposed a team up.  Onslaught promises Shinnok the ultimate prize, all reality to rule over after defeating every realm in Mortal Kombat.  In exchange, Shinnok will give Onslaught his powers back and more.  Onslaught will have the ability to bend and even break all the MUGEN barriers, reincarnate reality's evilest villains, and give enhanced powers to his chosen generals.  Unable to leave the realm himself, Shinnok commands his sorcerer, Quan Chi, to open a portal to the Marvel dimension.

            The Elder Gods notice the separation of Shinnok's powers.  By the time Raiden catches on to their plan, it is too late.  Onslaught has merged with Charles Xavier once again to transform into his more powerful form.  Onslaught wastes no time and bends the MUGEN barriers, Raiden delivers his message to all the dimensions and chooses the fighters to save all reality from Onslaught and Shinnok.

Let Mortal Kombat begin…