The Outsider and the Dragon are the other two remaining C'tan that are known of. The Outsider is commonly refered to as unstable and insane (not that any of the C'tan seem particularly sane), and the Dragon is a bit more of a mystery. GW has lots of vague fluff and stuff that seems to contradict on the topic of the C'tan, and on purpose I believe. Because so many Xenos have come into contact with the C'tan, and have so many different names (and because they do have lots in common with each other), things get confusing.
For example, one can of worms is the relationship between the Deceiver and the Laughing God. Also, has the Deceiver been posing as the Machine God to the Adeptus Machnicus, or is it the Dragon who really IS their machine god? Also, while the Nightbringer ate other C'tan, so did some of the other remaining C'tan.
So it all gets confusing and crazy! That's half the fun. =D
I'm finding it hard to design Necron characters or to even name my units for an army. This is my Necron Lord...his name is 1001000101. =)