Aether of Night is an unpublished, non-canon Cosmere novel. It is also the tenth novel written by Brandon Sanderson.
It follows a character named Raeth as he pretends to be his identical twin, taking over the throne of the Imperium and attempting to lead his people to victory against a foe they don’t understand and have little hope of defeating. “Aether” was a term used as part of the magic system.
The original version of the book was abandoned, as Sanderson felt that it didn’t strike a consistent tone between the almost Shakespearean comedy aspects and the utter destruction of the world. As a result, elements were incorporated into later Cosmere works, such as “Decay” becoming the Shard Ruin in the Mistborn series.
It remains possible that the book will be published in some form, but it would be a complete rewrite. Sanderson has mentioned that the general mechanics of the magic system would be retained. A substance called “Aether” appeared in Oathbringer, leaving the door open for the story to take place in the Cosmere.