It was humid there, and also very dark, so I once again summoned some firebird to have a better look, but to no surprise, it was just a hole in the dirt stretching further into the darkness. I rushed forward, encountering a few more lizards. I slaughtered them, all the way until I reached a wider chamber with quite a bunch of them.
Basically, it was the whole structure. A tunnel and then a bigger space with quite a bunch of enemies inside. There were spiraling lower and lower, all the way to the boss room.
It was a proper cavern, illuminated by blue crystals hanging from the ceiling, which emitted a pale light. Most of the space was occupied by stalagmites, sticking from the ground like the teeth of a giant best. Among them, in the very center of the space, was the boss. A snake-like creature covered in dark brown scales.
It was quite fat. At least a meter in diameter, maybe more, but I couldn't really tell with how it placed its body.