For the past year I've been running a multi-game line campaign set in the Jericho Reach, we started with Only War, had a few sessions where they played the Storm Wardens that arrived to clear out a Tyranid infestation (in addition to their IG characters), broke a siege, defended a planet, fell to Chaos, turned traitor got sold out by their Commissar and imprisoned. Mutinied and took command of their prison barge (by that time I was had everyone convert over to Black Crusade). Now we've been playing Black Crusade for about 5 months and they are making to conquer a section of the Jericho Reach. They recent ran afoul of Necrons which gets me to my question.

I decided to buy the Outer Reaches, and just finished reading it. Would it be possible to negotiate with the newcrons as if they could capture some C/SM for them to use as hosts for their Necron Lord?

How would you go about doing this?

With Newcrons, absolutely. Their leaders retain a sort of personality, up to and including the pursuit of hobbies and "diplomatic" contact with individual humans.

http://1d4chan.org/wiki/Trazyn_the_Infinite

(yes, in spite of the source, that is a real character from GW's codex)

Something to keep in mind would be that the Necron will only ever see any potential allies as pawns , and they will make very little in the way of an attempt at even hiding this. I'd say contact initiation could be either circumstantial (the players have something the Necron wants), or intentional by your players sending messages.

Perhaps attached to a gift. By the message being carried by a live human being, a la Luke trolling his droids in Return of the Jedi.

Servoskull: "... as a sign of good faith, I bestow upon you my loyal servant Flavius, who is in prime health and sports a rather unique mutation of his lungs. I trust you shall find his services agreeable, however you may end up using him."

Emissary: "... uh? T-that must have been the wrong message ... you silly little head!"

*Necron Lord gestures to the human, who swiftly gets dragged away to the laboratories*

To be fair even oldcrons had some sense to them...

At the end of it all just remember that all Necrons are just machines and logic engines. They weigh up the options and take the best one looking at the odds so if it was a choice between splitting their forces to attack IG/Deathwatch or a load of nids they may avoid fighting you for a bit but never expect them to give a **** if you get shot to hell or bleeding out. However looking at your scenario I can't really see your idea working overly well. Why would a lord negotiate with a party for CSM hosts when he could just kill Deathwatch marines/IG and use them instead? They could easily obtain hosts without needing to negotiate with the likes of you inferior humans.

Outer Reach made me wonder how easy it would be for them to acquire live Astartes host bodies.

Nobody in the group has any way of learning the Necron language. But if they did, they have access to an archaeotech organ cloning facility and could produce an endless supply of progeneroids to create new C/SM. I don't know if the Necrons would ever bother learning the Gothic languages...well maybe this Trazyn fellow might. Who knows.

Right now, I'm thinking of going ahead and having them get raided by a Necron harvester, and depending on how that goes maybe an offer could be made. Probably not, considering that most of the group is well on their way to becoming Daemon Princes, and they want to crush the Achilus Crusade with their Demiurg and Tau allies.

I think the need for them to harvest was retconned out with Newcrons. Necrons can do Gothic language though depending what you believe. Oldcrons did have that (see Dawn of War Dark Crusade and Soulstorm) via a single representative to speak for them but they would not be one for a casual debate, it would be usually short and to the point

Making new C/SM would be ULTRA HERETICAL (which makes me think adding it to Deathwatch sections, maybe not the best plan good sir ). Creation of any group of marines is usually done by professionals and usually only on orders of the High Lords or someone fairly important. So if they are even thinking of trying something like this then it would be time to throw oodles of corruption their way for even considering it. It is also a very difficult process with lots of care and control and isn't something you can mass produce easily without some serious resources and genetic material to work with and the changes of them screwing up without a good Magos Biologus (sp) is high. The requirements to the Necrons would be far too high for them to be content with a marine or two every month.

I would still think they are more inclined to just kill them.