The guardian of the Undead Horde's capital is ASHGAN. Ashgan, bringer of plagues, was the Alkmaar his priest. He was ordered by Mortis to never leave the capital unprotected.

The Undead Horde's capital - D1

The Undead Horde's capital - D2

In Dark Prophecy and the expansions, a captured capital crumbles into ruins.

In Servant of the Dark, the backgrounds are different from Dark Prophecy and there is a different background for each capital. They are real nice. Try to capture a capital, if only once, just for the ambiance. The Undead Horde's background is the one I prefer.