I'm starting a Dark Heresy game and have questions about the Culexus assassins. Where can I find rules on them? In which book? Are there novels that discuss them and their abilities? It's an Inquisition mission with a Radical Inquisitor as party leader. I'm building the characters myself because my group gets squirrely and wants to play the exact same things every time. So I'm building them and letting them pick from the selection. Any help is appreciated.

Wrong sub-forum, pal. This is for Fantasy.

The role playing game. I thought that's what fantasy meant.

Dark Heresy has it's own sub-forums:

Probably you'll get better answers in the forums for Dark Heresy which I print screened above.

Warhammer Fantasy Roleplay is set in an medival-style world, with elves, dwarves, knights and stuff, a bit like the Lord of the Rings books.

Everyone's answer is correct, but I'll link you to the forums, and then answer your question for completions sake: Forum for Dark Heresy 1E and Forum for Dark Heresy 2E

The rules are actually given in Rogue Trader - The Navis Primer. I honestly find them a little underwhelming, but I still have one in my back pocket for when I finally snap and need to kill my campaign's Astropath.

For any 40K RPG you can find a master lookup for any creature (or weapon) you need here: http://www.40krpgtools.com/library/