I am collecting and painting a Soul Drinkers army right now. I'm somewhat glad I'm not the only one
If you haven't read the 5 books that have been released(maybe 6 I've been out of the
BL loop for a couple of months) you'll find that they were corrupted by chaos mutations but turned back from chaos in time to save their souls. Their geneseed is still largely corrupted but after finding a cure they have just started recruiting more scouts, many of which die in a coup by some of those very scouts. They are low on vehicles, have only about 5 sets of terminator armor, no dreadnoughts and very few assault marines after their veteran assault marines turned into followers of Khorne. While this makes it very hard to compete on the table top if you stick to the fluff, I find it fun to base your tabletop tactics off of the established canon of your chapter. I run a scout oriented army and use the characters from the fluff to flesh it out(Librarian as Sarpedon, Techmarine as Lygris, Asssault sergeant as Graevus with no jetpacks, Dreo counts as Telion). If you don't want to do the conversions(which I suggest using chaos spawn/possessed parts for) then you can simply call them Soul Drinkers pre mutation. Good luck with your army, if you take the time to read up on them they are very flavorful.