From what I read Asorah is supposed to spawn some dragons. That does not happen though. Not sure if it's a inter mod issue though as I also have Rough Mobs installed (which maybe is the reason why the wasteland is so full of zombies and skellies).

That's never been a thing. Way back the dragons spawned more frequently (compared to them pretty much never spawning now with their spawn rates obliterated), making them a thing to keep track of while fighting Asorah. Since they no longer spawn at the frequency they used to, they don't really contribute to the fight at all.

Ah ok, got it. The book says "uses spectral dragons" so I assumed he spawns them. But that explains it then, thanks! (there is no way to up their rate again via config, is there?)

There isn't anything built in to change the spawn rates (then again, isn't there a billion mods that allows you to alter spawn rates of things?). The use part is that Asorah drains the health of them to heal himself.

Yeah, I misunderstood this part :-) And I will try to change spawn rates with rough mobs then as I am not sure there are really that many mods out there which can control spawn rates in 1.10.2 or I overlooked them. So thanks again.