I'm not sure if anyone mentioned this but i think its obvious that its Sasuke merely synching with Nagato's once used technique to summon the King of Hell statue. Madara said that he would once synch with it eventually once he knew of Sasuke's loyalty. But it does look more like the dark essence of that statue more than anything. It never did mention that it was a Rinnegan technique only.
Karin's line about Sasuke's dark chakra, and how Madara said Sasuke was "violated to the core by the shinobi world's hate and betrayal" was very similar to Nagato's/Pein's belief system and near the same exact words he thought of what changed him to Pein. [http://read.mangashare.com/Naruto/ch.../page010.html]
So it would seem in essence, Sasuke is becoming the next Pein in a way. I'm not saying that he'll have a Rinnegan or anything, but he'll simply become the sync between The King of Hell statue and the Akatsuki. With Sasuke becoming as evil as he is now, or at least set out on what he believes is the betrayal of the shinobi world particularly with how he's after Danzo for his manipulation of his brother and the eradication of the Uchiha family, i think its likely thats whats going on.
Sasuke is simply becoming the new host of the King of Hell statue
look at the similarities here between the two:
http://read.mangashare.com/Naruto/ch...0/page015.html -- sasuke and the shadow. look at the arches of the shadows back, its exactly the same as the King of Hell Statue
and the statue here: http://www.facebook.com/profile/pic....4_NiJHPc1tl2zt
HERE's ANOTHER PIC TO PROVE MY POINT!!
P.S. despite the eyes, which is a physical material, in a spectral realm... i still think its the Gedou Mazou.