Well, the extent of Stoick's regret varies in different fanfictions. Some authors focus a lot on his inner turmoil and the depth of his remorse, while others might just touch on it briefly as part of the story.
First off, you need to have a clear idea of the story you want to tell. Pay attention to details about the Whispering Death's abilities and the world it lives in. Also, create engaging dialogue between the characters to make it feel real.
To write such fanfiction, first, study the original character of Whispering Death closely. Understand its personality and abilities. Then, think of a compelling plot that showcases its strengths and growth. You could also add unexpected twists to keep readers engaged.
Hiccup's death in the fanfic could be a shock and create a lot of tension. It might force the other characters to step up and take on new roles, or it could even serve as a turning point for the entire story's theme and message.
One popular theme is the further adventures of Hiccup and Toothless. Writers often explore new lands and challenges that they might face. For example, they might encounter a new type of dragon with unique abilities.
You might consider creating a storyline where a powerful storm with lightning strikes causes chaos, and during this chaos, a mysterious force related to death itself emerges. Or perhaps have a character gain the ability to control lightning but at the risk of losing their life. Also, a quest to find a relic that can prevent the arrival of death itself could add an interesting twist.