(Spoilers All) Any theories on the Doom and the downfall of Valyria.

I have been wondering about Valyria and what caused its destruction. I have seen some theories here along the lines of asteroids or other celestial bodies, and have been wondering what other ideas people have on it.

Personally, I like the asteroid theory, however, it does not explain the "demons" or why people cannot go anywhere near it if they want to survive.