I'm guessing most of you fluff hounds have already seen this, but it was new to me. Basically some fellows over at the bolter and chainsword forums came up with an alternate timeline to the Horus Heresy where Horus resisted the temptation of Chaos and it was Rogal Dorn instead who was corrupted and led the Heresy. It's a fun little read.

http://www.bolterandchainsword.com/i...owtopic=200543