Well I was either going to have you square off against chaos in the Black crusade or against Orks in the third war for armageddon. I chose the black crusade and the astartes Praeses chapters seemed like the perfect list to pick from. I wanted to avoid using any chapter too well established as people would moan and bitch about being forced into playing chapters they weren't absolutely in love with. 

This way the players have the freedom of personality types and some lore can be made up in the game. Figured that might be fun for the more involved and 40k versed players to engage in. Obviously don't start naming chapter masters or thousand year old earth shattering chapter secrets without pitching me your lore idea first. 

Plus black and red AND a spiked skull. Super badass.