Maybe this is just me, but I like to think someone from Nether Gate, likely one of those missing bigwigs, wound up in Highland and taught a certain infamous Prince his bloodthirsty tactics, since I really doubt Han taught Luca to murder villagers as pigs.
That seems unlikely. Oboro has mentioned in the game that when he was part of Nether Gate, they never would have recruited someone as bloodthirsty as Childerich. Most of their assassins are methodical, not sociopathic.
As for their future, I get the feeling they're pretty close to extinction. Or have at least gone underground. It'd probably take some time to rebuild to their former greatness. And with being hunted by so many people, especially someone like Oboro with all his resources, it'd be very hard to remain hidden and build up your forces.