To build off of
my post in the Future of the Fortress thread, I'd like to suggest a path for all of these necromancers who find fortress life so interesting that they
must drop everything and visit. Blatantly revealing they're necros in the process. Right now, they're another useful set of hands and can be made to use their magical abilities in defense of the fortress and even turn an entire fortress into an immortal place full of what would be undead horrors were it not for the retaining of previous loyalties.
My suggestion is to turn this currently buggy behavior into a full feature in the future. Treat them like vampires and have them hide their true nature under a fake name, but instead of trying to sneak off to feed on other dwarves, they'll instead try to sneak off to corpse piles and crypts to steal dead bodies. From there, they could either cart them off the map (after which the player gets an alert that a corpse was stolen) or store the corpse in an assigned room for later use/theft. Now, they do of course lack needs much like vampires do, so trying to spy a hidden necromancer would be similar to trying to find the former. Maybe their pathing could be tweaked to favor corpses of say sentients then larger creatures or even megabeasts if a player has one stored somewhere, though the latter would slow them down massively as a result and give them away almost instantly I imagine. But that's part of the fun, I say, particularly if the player has to manage a larger fort at the same time.
The reason I suggest the storing bodies behavior is so, in case they're found out via dwarven justice, it adds a possibility that they'll raise the bodies they've accumulated and create an undead threat from inside a fortress. May be a bit too much !!FUN!! but I figured I'd put it out there for people to share their opinions. And of course, seeing as necromancers build villain networks, as villainy gets expanded upon, necromancers could also send their apprentices to explicitly retrieve bodies secretly from a player fortress if they have any. And interrogations can reveal which tower they're coming from so the player can send retaliation in the form of raids.
Of course, that's not to say some things should be removed, I honestly think the fact players can use this behavior to create undead fortresses is pretty cool too. So perhaps an undead fort, with intelligent and non-intelligent undead roaming around, could be a possible outcome of successful repeated necromancer infiltration of a player's fort. And the player could either allow it and manage the undead with the necromancers taking charge, or lead the resistance of the last living dwarves against the necro overlords. There's many possibilities with that really.
If anyone has any other suggestions or alternatives, I'd love to hear them.