What do I need to field and assassin in a Warhammer 40k space marine army?

Question

I have a space marine army in the latest edition of warhammer 40k, but I haven't played properly since 2nd edition where you could just use an assassin in any imperial army. If anyone could tell me what are the requirements to field an assassin now, I would be very grateful

Answer 1

You need an Inquisitor in your army, To be able to pick one of the assassins.  

Answer 2

you still can take an assassin.  Rules for the assassins are in the daemonhunter (and witchhunter iirc) codex, as well as rules for taking units from the codex as allies in space marine or imperial guard lists