User blog comment:Sinister 00/About 666/@comment-82.40.154.8-20140103231514/@comment-16281343-20140103233450

The connection between Rizevim and 666 isn't established yet. The only spoilers only revealed 666's existence and none are said that Rizevim wants to free him or anything.

The reason why Rizevim wants Great-Red dead is for him to be able to go to the place where Chichigami is and conquer other worlds.