The Radiance: Draining "entropic" energy
by Bruce Heard<< What type of rules would you use for draining Entropy out of the world? >>
Draining "entropic" energy would only serve to physically weaken the power of related creatures (undead, fiends, etc), at least one would think at first. But it's really a catch-22 situation. How does one destroy the forces driving "destruction"? How does one kill "death"? The drain would merely be a source of pain to creatures serving entropy, and as a result they become more active in their dark, nefarious ways.
For example vampires would experience an even more painful thirst for the blood of the living. Fiends would crave even more new, untainted souls. Other creatures of the night would leave their lairs and band together to reap new crops of death and pestilence from the world of the living. In fact, the drain on entropy just connives to make its denizens much worse, driving Mystara toward another disaster. Instead of losing its magic, Mystara would witness its lords of entropy become an even more terrible threat.
You can interpret this approach in two ways. Either it is a status quo -- the magical drain stops, but evil just remains evil -- OR the balance of things tip in a different direction altogether -- lost magic is slowly regained, but you now need it even more in your desperate struggle against the unprecedented predations of entropic minions driven mad by their pain. It all depends on how you want to run you campaign.
Mechanics? If you need specific mechanics, roll d6-1 hit dice for entropic monsters rather than the usual d8 (or more simply, subtract 1.5 hit points per Hit Die as appropriate). These monsters would experience a permanent state of "hunger" and do whatever they normally did (per original monster descriptions) but now in attempts to regain these lost hit points. For example, shadows may choose to attack living creatures, draining their victims' Strength to regain their own depleted hit points, and leave as soon as their "hunger" is satisfied. Other cruder beings would more simply need to "eat" living flesh to achieve the same effect (zombies, ghouls). The tougher the creature, the more subtle its way of replenishing lost hit points. Points regained in this manner only last until the next new moon, after which they drain away again. So every new moon (on Mystara) or at the time of some regular event elsewhere (other planes for example), servants of entropy all suffer the same pain and a burning desire to seek a new retribution.