I'm not sure I like the ideas of ghosts.
I do however like the idea of dead creatures in an evil biome having a chance to reanimate if they are not in a tomb. Make it maybe a 10% chance per month? Something like that, the numbers can be adjusted. This means that if you have a bunch of non-butchered, non-entombed corpses somewhere they will eventually rise up as zombie or skeletal versions of themselves.
A butchered corpse is taken apart into many tiny pieces so thats not an issue. A corpse in a coffin is secured, so thats also fine. Its the things sitting around in your refuse stockpile, or perhaps you don't want to clean up after a siege, which means you'll get some undead goblins rising up from the bloody mess if you don't handle it.
You could even exploit this to create your own undead mosh pit. Toss a bunch of corpses into a pit. The corpses will have a 10% chance per month to reanimate, so eventually all of them will be active again. Toss anyone you don't like into the pit. The undead will tear them apart, and then Urist McUseless will rise from the dead to join the undead. You could even use this as an archery training range. You can't kill something thats already dead, all you can do is knock them down for a while, but the undead have a disturbing tendency to get right back up again unless the corpse is destroyed. Butchering it will do this, but simply putting some crossbow bolts into it won't.
An undead pit would make for a horrific sort of trap. Make your entrance have drawbridges over the pit of the undead. Drop a goblin siege into the pit. See how well they do against the army of the undead! They will of course eventually lose. Even if they win at first, those corpses will stand up again. And again. And again. All slain goblins will also stand up again. And again. And again.
This could be so much
FUN.