I know it's not Canon, but has anyone come up with rules for playing Necron, and if so, how would they incorporate themselves in a warband? I could imagine a Necron styled after Legion from Mass Effect being quite interesting.

Ive thought about it but it isnt high on my priority list to actually do. It would need to be a higher level necron type than warrior though, as warriors are lacking much of thier former personality and volition. An immortal would never have a realistic chance of realizing ambition with other necrons either. I would try to go with a lord or cryptek. Maybe have a lord as the sole survivor of thier tomb, and the have decided to use the humans to find another tomb to take over and begin building his own dynastic house. The cryptek could be traveling with those who have souls to study them in order to learn more about biotransferance. Studying the sea of souls, sometimes through thier eyes. You could have their memories/experiences damaged in the long sleep so don't start with all the skills they might have. Rather than specials they get to use necron tech without penalty, take four cryptek scarabs as a single minion of chaos, and one cryptek wraith as a minion of chaos, maybe a cryptek spider as a greater minion. They must still purchase those feats before they can take them as minions.

I had my PCs helping a Necron Lord for a while. He used to be a Lord but was betrayed and improsned. So you could use that as a story line.

As for traits/skills look at the Necrons in the Tome of Fate.

You should read the short story Duex Ex Mechanicus by Andy Chambers (originally published in Inferno! Magazine several years ago, reprinted in the Black Library omnibus book Let The Galaxy Burn ). It features- SPOILER WARNING!- a Necron infiltraitor in an Adeptus Mechanicus explorator group (needless to say, things end badly for the servants of the Machine God…). That would be an interesting twist to a Black Crusade campaign: one of the heretics in the PC warband is secretly a disguised Necron, persuing his own agenda- and since Necrons are hard-core anti -Chaos, that agenda will almost certainly result in the eventual Total Party Kill of the group if successful…

Sounds like the perfect guy to have on your team, my Alpha Legionaire would use him to dispose of any allies who'd outlived their usefulness, at least until it was just the two of them left (everyone knows that Necrons can't Self Terminate). lol