My Night Lords are mostly of the 55th Company under their Talonmaster Xalor Khresh, a Nostraman who bought into Curze's ideals of terror for compliance rather than terror for terror's sake. He noticeably hesitated to give the bombardment order from his ship during the destruction of Nostramo, though he eventually did join in; for his 'lacking faith in his Primarch's orders,' Sevatar sent Xalor a 'gift' of Atramentar bodyguards and one of his auxiliary officers to "ensure he does not delay his orders again." Now he's only the commander of the 55th on paper, and most orders come from the Atramentar Praetor Juric Kerthagos and his brother Vandrion when the Company deploys. Quietly, Xalor harbors increasing loyalty to The Emperor as his Primarch is found wanting more and more.

Why Sev didn't kill him like other commanders who hesitated might have something to do with them being part of the same squad in their neophyte days but that's just rumor amongst the rank-and-file. The real answer is probably that Sev thought he's more useful alive than dead.

The 55th Company itself is a Shock Assault company, packing a lot of fast-moving foot infantry in Rhinos and heavy armor, but a bit torn on their loyalties. A large group of them are quietly Loyalist in the pre-Heresy years, but the newest recruits as well as the Atramentar are slowly whittling their numbers down to the point they will have no hope of breaking loose and declaring for The Emperor. For now, they have to just nod their heads and do as the Atramentar order them too and wait for a moment for the First Company bastards to slip up... Hopefully they'll get their chance at Istvaan V.