One great 'Shadow Monarch Fanfiction' is 'The Ascent of the Shadow Monarch'. It has a really engaging plot that follows the main character's journey in a very detailed way.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of 'Shadow Monarch Fanfiction' with different ratings and tags to help you find what you're looking for.
It could be an original character. For example, a young orphan ninja who has always felt like an outcast in the village. Discovering the Shadow Monarch powers gives them a new sense of purpose and a chance to prove themselves.
One idea could be a crossover where the Shadow Monarch's powers are introduced into the Naruto world during a major conflict, like the Fourth Great Ninja War. The main character, perhaps a young ninja, discovers the power of the Shadow Monarch and has to learn to control it while also dealing with the existing political and power struggles in the ninja villages.
Some fanfictions might have a plotline where the shadow monarch powers are linked to an ancient magic in the Harry Potter world. Maybe it was a power that was once known but lost, and now it's resurfacing through the main character. This could lead to a search for the origin of these powers, uncovering long - forgotten secrets of the wizarding world and changing the power dynamics between the good and the bad in the fight against Voldemort.
Harry Potter himself is often a popular character in these fanfictions. Since he's the main character in the original series, it's easy to incorporate him into the shadow monarch concept. Another popular one could be Hermione. Her intelligence could be combined with shadow monarch powers in really interesting ways in fanfiction.