You'll have to kill Vampires, I think they are in the tenebrous mountains. I may be wrong, but If I remember the vampires have a faction modifier when you kill them.
I am a Level 46 Troll SK and I went to Katta Castellum to get a bind and start my darkened knight armor quests. I am an ally in Shadow Haven But to my dismay...I am KOS in Katta Castellum. How do i fix this Faction. I am on Morell Thule server.
They're good xp 57+ (level they become single pullable)
But if you insist, I think killing Seru guards (every katta guard I've killed thus far has given + a bunch of seru faction) or coterie in Tenebrous (+coterie of the eternal night from killing guards)