I was just looking to read about the C'tan, I ain't trying to marry one. 
Since they (along with the Necrons) were drastically changed recentish, this seems like a good time to tell you that there is no canon in 40K, so research to learn about the universe is largely a waste of time. It's for things like identifying what Mk VI power armor looks like instead.
This is why I told you to read Lexicanum instead of the Wikia, it's much shorter (and thus won't waste as much of your time) and much more conservative in what information is allowed to enter the encyclopedia.
Since you're reading about teh spehs mehrunes dagon, check this out.
Astartes (spehs mehrines) are the sloppy seconds created by the Emperor after his genetically engineered knights of the Round Table (called primarchs) were stolen from him.
Now when this happened ca. the year
2,000 30,000 humanity was emerging from a long dark age (not that it still isn't in one in M41) and technology was not particularly sophisticated, so how could he have done all this? The story used to be that he was just that awesome, which isn't ridiculous for 40K.
Well, turns out that he actually (probably?) made a pact with the Chaos gods to acquire the power necessary to create his smartdumb knights instead of just being awesome.
If you never read a book from the Horus Heresy (Mordred's rebellion, to continue the Arthurian metaphor) series you'd never know this because any book is limited by the perspective of this characters in it, even omniscient narrators aren't, it's just some stupid Imperial scribe limited by the time and place of his life.
This is also why everyone has ridiculous diction and the writing is equally if not more so.
