D&D 5E: Fantasy #30: Hellpits of Nightfang

The Hellpits are a trio of natural sinkholes near a busy trade route and a wilderness town.

The Hellpits, and the natural caverns beneath, are now the lair of an ancient evil. The vampire Nightfang, and its forlorn allies, now dwell there, selectively preying on the travelers and populace so as not to draw the attention of the forces of good.

By what other secrets of a bygone age are concealed in the Hellpits?