I'm not entirely sure if there is a specific 'Harry Potter kills the Night King' fanfiction. However, given the popularity of both 'Harry Potter' and 'Game of Thrones', it's very likely that some fans have been creative enough to write such a story. Fanfiction communities are full of these kinds of crossovers, but you may need to search through various fanfiction platforms to find it.
A good fanfic would have strong character development for Ayra. It should show her growth from a normal girl to a powerful assassin capable of taking on the Night King. For example, her skills in combat and stealth should be well - described and progress throughout the story.
Well, Harry Potter would likely use his powerful magic. He might use the Avada Kedavra curse to kill the Night King. Then, the magical world of 'Harry Potter' would be intertwined with the world of 'Game of Thrones'. Maybe the wizards would then have to deal with the aftermath in Westeros, like helping to rebuild or dealing with the power vacuum left by the Night King's death.
The origin of the Night King dates back to when the First Men and the Children of the Forest were in conflict. A man from the First Men was turned into the Night King by the Children of the Forest. This was a strategic move on their part, but it backfired horribly. The Night King then started creating more White Walkers and the undead army. He was no longer under the control of the Children of the Forest. His power grew and he became a terrifying force, bent on bringing an eternal winter and darkness to the world.
I'm sorry, I haven't got any information regarding the author of this 'King of the Night Novel'. It could be an indie author or from a small publishing house. You could try searching for it in book databases or literary forums.
In some fan - made stories, the Night King is often depicted as an even more powerful and ancient evil. He might be a force that once threatened other realms or dimensions long before the events in the main 'Game of Thrones' story. He could be a being created by a different kind of dark magic or be a part of a completely different mythos within the fictional universe.