http://www.uesp.net/wiki/Lore:Azra_Nightwielder is said to be the first person to discover shadow magic and to be the greatest of the shadow mages. The http://www.uesp.net/wiki/Shadowkey:Scroll_of_Shadow defines shadow magic. This http://www.uesp.net/wiki/Shadowkey:Brown_Scroll refers to shadow magic in passing.
However, Azra Nightweilder and both of those texts come from the portable game Elder Scrolls: Shadowkey. This http://www.imperial-library.info/content/summary-reaver gives a lot of shadow magic references. It appears the idea of shadow magic was introduced in Shadowkey and then stayed in Shadowkey. If this is the only game where it is mentioned, can shadow magic even be considered Elder Scrolls cannon? I can find no mention of it in the four main games: Arena, Daggerfall, Morrowind, and Oblivion.
I am searching for lore on which I can build a rather extensive mod. While the idea of shadow magic is interesting, it seems fishy that this entire brand of magic is never once mentioned in the lore of any other Elder Scrolls game.