A great fanfic is 'Beyond the Shadow's Veil'. In this one, the story explores what lies beyond the typical scope of the Eminence in Shadow's activities. It has a great plot that keeps you hooked as it reveals secrets and new aspects of the shadowy world.
You can start by looking on popular fanfic websites like Archive of Our Own or FanFiction.net. Just search for 'Eminence in Shadow reaction fanfic' in their search bars.
Some people find Eminence in Shadow manga on dedicated manga websites such as MangaFox or MangaReader. But be careful as some of these sites might not be legal or have reliable content.