As mentioned depending on the mud type you are running, this could be anywhere from easy to moderate. With Diku-Derivs alot of them have *progs which allow you to use a generic scripting language. I know in Circle, DGScripts has a trigger suited quite nice:

Reset
Description: This trigger is called each time the zone of the room it is attached to is reset.
Argument: Not used.
Numeric Argument: The chance this trigger will run when it is checked.
Return value: Not used.
Variables: None.

If you are planning on using other than a Diku-Deriv then it can be easy too.. in an LPC based mud it's quite simple, just a nested switch in the create() of your room. Abermud and Mordor would probably have to be hardcoded (unless you have pDirt abermud)..

Kaylus@Solice