The Zoo
75 years ago, the Order of the Light of Pelor opened a new training facility in Mt. Daneasus: The Zoo. Located in a somewhat remote location down a single well-defended tunnel in the lowermost reaches of the mountain, it was a giant oblong facility surrounded on all sides with a positive energy field. This field, and various other defenses, contained a menagerie of the walking dead - to study, train against, and destroy.
If the idea of keeping a horde of terrifying monsters in the most populated city in the world seems like arrogant folly, that would be correct. Because eventually, the defenses failed. The menagerie was released. And it took some of the greatest heroes of the day to protect the population.
The Zoo is no more.