Night Lords could be interest to be involved for various reasons. Perhaps they've been offered some payment for their involvement in Arkhona, perhaps there is something of interest to the Night Lords or perhaps this group of the Night Lords hungers revenge against the Dark Angels during the events of the Horus Heresy. Who knows why their interested here. I don't want to get close enough to a Night Lord for him to explain his heart and mind as he'd probably rip out mine... Dark Eldar might have been convinced to take an interest by their Eldar cousins or the Harlequins if there is something there that is of a great enough threat to both of them. Unlikely though. Have Dark Eldar been confirmed?
I think you underestimate the Night Lords. Sure they like letting out their sadistic side on civilians, but they can also fight and fight well. As for Dark Eldar, from the lore it appears that there is something of great value and probably great power hidden in Arkona. Every cabal will be working to secure that power, and also working to make sure the other cabals do not.
They actually do, because are a Space Marine Legion that's been fighting a ten thousand year war and inevitably will produce Warbands that are into that, just less frequently then ones who aren't.
uhhh no...the whole point of Night Lords is being cynical anti-religion thugs and the Alpha Legion who fall to that kind of level no longer use the name Alpha Legion, they become groups like The Faithless who have become slaves to Chaos
Chaos and the Legions are not set in stone things, certainly not after the slave wars. Night Lords who practice ritual sacrifice could totally be a thing, the doctrine of using 'fear as a weapon' does not intrinsically exclude the Chaos Gods, they might not be popular, but they are there. There's no 'high command' that's going to tell them "No, you cannot call yourselves Night Lords now." after all.
Next thing you will say that Word Bearers could totally become Atheists right? I dont think you understand the psychological effect of a Primarch on his sons, they dont change, they ARE like their father
Warbands can actually differ from their main source in quite a variety of ways. Yes, the Night Lords go in for all of the above and the psychological effect from their Primarch definantly sticks good, but there's always change. Wether it's unsatisfaction with the Imperium or Daemonic promises, it happens. There's no sign of Night Lords or any of their Warbands conducting rituals or anything like that and the mindset on the Warp from their Primarch has definantly stuck on most of the Astartes. Yet with their savage and cruel nature, I'm lead to believe atleast some of them has changed in such a manner as Darthy said. There's always Acerbus, he's a Night Lord Daemon Prince. He's definantly done something that doesn't quite comply to the Night Lords mindset. Best part is he leads a large Warband. I think there's something in that warband, involving sacrifice and whatnot.
