I settled near a tower and got some undead hordes passing by every now and them. But it seems some of the necromancer liches chose to linger around the map for a few years now. They are completely invisible, I can only see them through combat logs, when they occasionally raise corpses and throw some very nasty spells at whatever passes by. They've dealt with most of the other ambushes and sieges, and killed off enough caravans and diplomats that I'm now at war with everyone.
I tried sending my military at them, they had no problem dealing with the undead but once they even got close to one of the liches they get turned into undead and start killing eachother, get necrotized, cut into pieces and the lich is still invisible.
I created an "arcane protector" to see if he can do anything, the lich doesn't seem to cast spells on him but he's still invisible. He also couldn't handle an endless stream of undead, the white aura didn't seem to do anything.
There seems to be only one solution - flood the world with magma, and a plan for this is currently in progress.