The Sylvan Temple

From the introduction:

Some two hundred years ago, a group of elves from the Arcaskdry tribe split from their folk, so as to establish a cult to a demon who had revealed himself to them as a savior god of the elves. They travelled to the small woods (now known as the Vale Woods) that were just to the west of the Arcaskdry Forest, and built a large, tree-shaped tower to serve as a temple to their new “god.”

[...]

The temple has stood since, slowly falling into ruin. Since the settlement of other races in the area, and the founding of Neathby, a handful of adventurers have explored it, some returning with riches, others never to be heard from again. The last successful excursion into the temple happened a few generations ago.