Well, in a fanfiction scenario, Harry Potter might use his powerful magic like the Patronus charm. The Patronus, being a powerful positive force, could potentially drive away the Nazgul which are associated with darkness and fear.
The Shadows of War Nazgul stories are mainly about the dark and menacing Nazgul in the game's lore. They are powerful servants of Sauron, and the stories might cover their origin, their powers, and their role in the war.
The Nazgul were once men. They were corrupted by the power of the Rings of Power given to them by Sauron. These men, kings, sorcerers, and warriors in their former lives, became his most feared servants. Their back stories involve their initial allure to the power of the rings and their gradual descent into darkness under Sauron's influence, losing their humanity bit by bit until they became wraith - like beings, bound to Sauron's will.
One idea could be a Nazgul crossover with the modern world. Imagine the Nazgul appearing in a big city, causing chaos among the unsuspecting population. Another could be a crossover with a fantasy realm like Narnia, where the Nazgul's dark presence contrasts with the more magical and positive elements of Narnia. A third idea is a crossover with a superhero universe, and the heroes have to figure out how to deal with the Nazgul's otherworldly powers.
One interesting Harry Potter and Nazgul fanfiction could be one where the Nazgul somehow cross into the Harry Potter universe. Maybe they are drawn there by the dark magic that Voldemort wields. A great place to start looking for such fanfics is on fanfiction.net. There are many works that blend different fantasy elements together in creative ways.
One of the most important aspects is their fall from grace. They were once men of some standing, but through their greed for power via the Rings of Power, they became slaves to Sauron. Another aspect is the way their physical and mental states changed. Their once - human forms deteriorated into something more sinister, and their minds became completely dominated by Sauron's will.
Here is a simple fanfiction: Harry and Frodo met in a strange magical forest. Harry was on a quest to find a new magical artifact, and Frodo was still carrying the One Ring. They decided to travel together for safety. Along the way, they shared stories of their different worlds. Harry told Frodo about Hogwarts and magic spells, while Frodo told Harry about Middle - earth and the great battles against Sauron. Their journey was filled with challenges and new discoveries.
Frodo's life was a sad story because he was a simple hobbit from the Shire at first. He was happy there, but then he was thrust into this epic adventure. He had to endure long and arduous travels, face terrifying creatures, and deal with the psychological toll of the Ring. In the end, even after achieving his goal, he couldn't fully recover and had to leave Middle - earth. His life was forever changed and scarred by the events he went through.
You can start by looking on fanfiction.net. It has a large collection of various fanfictions, including those related to nazgul crossovers. Another place is Archive of Our Own (AO3). It's known for its extensive range of fan - created works and has a good chance of having some great nazgul crossover fanfictions.