Instead of instant death to the living, how about the Shroud impose the fatigued condition and drain 2 Constitution points per day, with no way to magically heal it within the Shroud?
The fatigued condition puts the living at a severe disadvantage against the undead locals while making it possible to have adventures there.Fatigued
A fatigued character can neither run nor charge and takes a –2 penalty to Strength and Dexterity. Doing anything that would normally cause fatigue causes the fatigued character to become exhausted. After 8 hours of complete rest, fatigued characters are no longer fatigued.
As for the Shroud itself, its likely that the remains of the Albemarl Device is the font of negative energy for the Shroud, and entering The Grim Fastness and destroying it would likely bring down the Shroud.
One other matter - its not clear how the undead with feeding requirements in Necropolis are even able to survive. They would have depleted the mortal population within easy reach, so they remain in existence by some other means. Most likely it is the Shroud that supplies the necessary "nutrition" needed for these undead to remain unalive. While the Shroud keeps them unalive, it does not keep the feeding undead from hunger, which they experience always because there is so little "food".
Infiltrating Necropolis, destroying the Albemarl Device, thus bringing down the Shroud would be a great adventure to culminate in a street by street battle of Darkon's forces vs Death's legions. It can be the Ravenloft version of the Red Hand of Doom, with victory points and so on.