Magic is the lifeforce of all living creatures, and it is from it that all life is made - in areas with lots of undead, magic might actually be missing or weak, which puts a whole new spin on how dangerous the undead are. Even simple zombies can be troublesome if you're relying on mages and you hit a pocket of undead making a low-magic zone...
"...the Ilthian mountains. A craggie masse of rock rysing from the Ilth'n plaines. The waters whych springe from it are ful of godeness and fortyfie those who drynk them [+1 STR]. Alas the vyle beasts resydent in these hills also bathe in these waters, and in the doing gain great strength. Foes mortallie wounded have bene known to flee, onlie to return, revytalised houres later..." - Chronicler Eamusil, Mondopedia, Vol XV (The Lands of Sylmen)