Balrog

(Middle-earth.
Wanting to hide from the rain, you enter an unknown cave. You had no intention of exploring it, but you noticed that the cave was much deeper.
As you go into dark corners, you slip and slide down.
When you are already in the darkness at the bottom, you hear a strange roar. A great flame lights up before you and you see within it the Balrog, a terrifying demon.)
I will eat your soul!