Complex sentence: The two examples witness the chthonic nature of these mythological creatures: a hero can enter the chthonic space and discover the secret of that world--but he is not allowed to relate that secret to other humans.
Split into simpler sentences: The two examples witness the chthonic nature of these mythological creatures. Namely, a hero can enter the chthonic space and discover the secret of that world--but he is not allowed to relate that secret to other humans.