I actually thought Outcast Dead was the best of the bad books (Vulkan Lives, Fear To Tread, Battle of the Abyss, the first Dark Angels books). It was interesting, but too much of it didn't make sense (such as a naked marine being able to punch through a Custode's power armour. The same guys who can walk through possessed Marines, and Chapter Masters like they're butter, get dropped by a single World Eater. And Horus lost?)
I thought it had potential to be good, and i wanted it to be (in parts it really was), but i lost it when the time lines got confused. It's my understanding that Prospero and Istvaan happen in a matter of weeks from OD, whilst there's meant to be about 5 years between the two.
But hey-ho. I've heard some theory about the warp messing with time, but i don't really buy that theory.
Having said that, I loved both Samurai Jack, and the Gang-Boss. They were pretty cool characters, of which i'd like to see more.
Also, Harlon Nayl. I'd argue he is an obscure character, and he's pretty cool. As is Moloch, his character is so much fun.