Wraithform on a mummy is 100% bugged. As mentioned by a previous
mummy, every single dispel works against it. Even low level mobs
dispel it every time. Surely it is not by design that this spell
is dispelled with every attempt. People in game know this and are
exploiting this bug.

I figured out the issue, tricky though it was.

First I tried having necros dispel each other. No issue, everything was fine.

But then I got thinking about it being a "mummy-only" issue, so I played some more.

The affect wasn't loading properly for undead so that it *was* becoming easier to dispel it.

That is fixed and your wraithform will be properly tedious for people to wipe out.

Do you undead scum need more power? No. But there you go! :)