Part 4 of 5, Gray Huntress and the Proto-Deus, by heozART:

It seemed there was no one around, but the door was open, so Gray Huntress let herself in. The interior of the church was almost bare, but she was strangely drawn to its simplicity. Curiously, the wall behind the altar was decorated with very elaborate relief carvings. Gray Huntress thought it looked familiar, and sure enough, the pattern on the third arrow was a perfect match for the carvings on the wall. Upon closer look, there was a hole in the center of the wall in which the arrow fit perfectly. The intertwined sections of the carvings started moving in all directions, and when it stopped, a set of doors opened before her.

What was waiting for her behind the doors was a secret underground complex. Technically, it would be inside the mountain. Gray Huntress wondered who could have built a structure like this. It was very dark even with a torch, but she knew it had to be massive from the echoes of her footsteps. (Don't ask. There is always a torch.) "Ok, mystic, I am not afraid of the dark. What am I looking for here?"