It could be that the character who is the Master of Death in the fanfic is seen as a prophesied figure in the Warcraft universe. Since the Master of Death has conquered death in the Harry Potter world, in Warcraft they might be seen as a being who can bring new life or restore balance. For example, they could use their power to heal the war - torn lands in Warcraft, much like how they might have used their power to protect or revive in the Harry Potter realm.
The Master of Death status might make Harry Potter a target in the Warcraft fanfic. There could be dark forces in the Warcraft world that want to either use his power for their own evil purposes or simply eliminate him because they fear his influence. This could lead to a lot of action - packed storylines, with Harry having to fight off these threats while also trying to uphold his values from the Harry Potter world.
One possible fanfic could involve Harry Potter, with his newfound status as the Master of Death, being transported to Asgard. He might use his skills and powers to face the challenges there. For example, his ability to master death could give him an edge in battles against Asgardian foes. Maybe he'd form an alliance with Thor, and together they'd fight against a common enemy like Loki who has some new, more dangerous scheme.
When writing a 'Harry Potter Warcraft Fanfic', it's important to do your research. Read both the Harry Potter books and play the Warcraft games to get a feel for the details. Plan out your story structure carefully. You might start with a character from Harry Potter being magically transported to Warcraft. As you write, build up the relationships between the characters from different worlds. Describe the new and strange environments they find themselves in vividly, so readers can easily picture it in their minds.
One great 'Harry Potter Warcraft Fanfic' is 'The Wizard and the Warrior'. It combines the magic of Harry Potter with the epic battles of Warcraft in an exciting way. The story follows a young wizard who gets transported into the Warcraft universe and has to use his magic skills to survive and find his way back home.
Perhaps an ancient spirit that has been awakened by the imbalance in the life - death cycle due to Harry becoming the Master of Death. This spirit sees Harry as the cause of the chaos and tries to destroy him to restore the balance in a way that is harmful to the wizarding world.
Obviously, Harry Potter is the main character as it's centered around him being the Immortal Master of Death. He would be a more powerful and perhaps more complex version of himself from the original series.
Well, Harry could find that his Deathly Hallows are perceived very differently in Asgard. His Invisibility Cloak might be seen as a powerful magical artifact like the ones in Asgard. His Resurrection Stone could be a source of great fascination as Asgardians have their own beliefs about the afterlife. And his Elder Wand might be seen as a weapon of great power, perhaps on par with Mjolnir.
A common theme is power and its consequences. Since Harry is the master of death, he has a great deal of power, and fanfics often explore how he uses or is tempted by it. Another theme is the blurring of boundaries between the known magical world in Harry Potter and the added supernatural elements.
One possibility could be that in this fanfiction, Harry Potter uses his mastery of death to have unique abilities in Skyrim. Maybe he can't truly die in the Skyrim world, and this gives him an edge in battles against dragons or other foes. He could also use his knowledge from the wizarding world to manipulate magic in Skyrim in ways that are different from the native mages.