Well, just from the name, the combination of elements like 'big black zahhak' and 'plumber' with 'equius' is likely to be unique because it's not a common set of things you'd typically see together. It could bring a very different and unexpected story, perhaps mixing different genres or concepts in a way that's not often done in more mainstream fanfictions.
I'm not sure if there is a very specific and widely known fanfiction with exactly that combination. You could try searching on fanfiction - specific websites like Archive of Our Own or FanFiction.net using those exact keywords to see if any relevant stories pop up.
I'd recommend checking out 'The Adventures of Nepeta and Equius' on Archive of Our Own. It has a great plot that really brings out the best in both characters.
One of the great things about Nepeta x Equius fanfics is the potential for character growth. We might see Equius becoming more in touch with his emotions through his interactions with Nepeta. And Nepeta could also learn about boundaries and self - control from Equius. Fanfic writers can take these two characters in many different and creative directions in their stories.
One main event is Zahhak's transformation into a demon king. Another is his reign of terror where he demanded human brains for the snakes on his shoulders. And finally, the attempts by heroes to defeat him.
Well, Zahhak in the story was a figure full of evil. He came to power through some sort of dark influence. His story is also about the battle between good and evil. Many heroes in Persian mythology tried to overthrow him to end the misery he brought. It's a complex story that shows how a once - normal being can be turned into a great source of evil by the forces of darkness.
One main theme is the clash of personalities. Equius is so rigid and proper while Vriska is more wild and unpredictable. Their relationship is often about how they deal with these differences.
Zahhak himself is the most important character as he is the serpent king. He is the center of the story's events, likely being the source of the conflict.
One main theme could be the battle between good and evil. Zahhak represents evil with his tyrannical rule, and there might be heroes or forces of good that oppose him.
There's a fanfic called 'Equius and Vriska: A New Understanding'. In this, it starts with their typical rivalry but then gradually shows them finding common ground. It's a great read if you like seeing their relationship evolve in unexpected ways.