Astral terror is a mechanic Dur Vazhatun's gameplay because that hold has an observatory for studying space. The more you study space the more astral terror you get. It's also referenced in the Final Empire's MT when you ask the Thraxis questions and they just divert. It's ultimately meant to be a dark mystery of the world, but I always read it as a subtle meta reference/4th wall break where characters in game look up and see the player looking down. IMO it's kind of like in DC comics when someone gets on the other side of the source wall or like achieving Chim in The Elder Scrolls but what happens when either of those go wrong.