Black Reavers are persistent, but kinda easy to trap, like Michael Meyers or Jason Voorhees. Assuming you have that much firepower to back it up, of course.
The worst villains are the really smart ones, like my personal worst high level nemesis was a high level evil priest of the god of death, who had the "Ambassador's Cloak". . . a constant artifact of non-detect and misfeel. . .which allowed said priest to sit on the cardinal's council of the god of order, and run his evil plots from the very heart of warded and defended friendly territory. . . .last place you'd ever look for the supervillian, eh, one of the guys sitting in the room inside the building that's warded so heavily that evil demons and evil priests burst into flame before they get there?
So I've had painful personal experience with the core concept behind this thread, and via magic items it doesn't just rest in the hands of nightblades.
Your party sounds super powerful, and it gets hard to challenge such a group, especially in a combat sense, without crossing the line into permadeath. . . .nobody likes to spend so much time developing a character to then lose them, and to really challenge this group, you need to put them on the razor's edge from dying vs some really nasty foes. . . I suspect nobody would end up happy from a TPK here.
The last game I played in with a group going into levels like the GM really needed to focus us onto non combat oriented problems, or "Save the lemmings" issues. . .i.e. get into a fight with a dragon in an urban area, so the PCs (assuming they have the values to feel compelled to) spend more effort trying to save innocents than directly fighting, and hold back a lot to stop from killing people themselves.
Also, more subtle, corrupting problems, like high level evil mentalist Greater Vampires who want set the PCs up to commit evil acts then deal with the consequences. (Between him, and killing paladins sent on orders from the evil priest up top, heroes rapidly destroy their reputations and become hated, hard to fight a bad reputation with martial prowess.)
The GM in that particular game was also quite happy to use our divergent goals to put us into opposition with each other, though we often found compromises, and some players will get very unhappy with the GM "Encouraging intra party strife".
It's a tough road, such a high level group, with a game that runs rarely for intensive sessions, since as you're discovering, it's very hard to come up with challenges on the table, coming at them in the open, that they can't step on via some combination of their powers.