Well, 'Dagon' is indeed regarded as the first Lovecraft story. Lovecraft began to develop his distinct style with this piece. In 'Dagon', we can already see his fascination with the unknown and the grotesque. The story's protagonist's encounter with the eponymous Dagon and the desolate, nightmarish landscape he finds himself in are hallmarks of Lovecraft's emerging horror aesthetic.
The fact that 'Dagon' was Lovecraft's first story can be attributed to several factors. Firstly, it was among his initial attempts at writing horror fiction. In this story, he introduced ideas that he would later expand upon. For example, the concept of ancient, malevolent entities that exist beyond human understanding. The narrative structure and the use of a first - person narrator to convey the sense of horror were also new elements at that time for him, making it a starting point in his creative journey.
It could be about the cults and the unholy practices associated with Dagon in Lovecraft's stories. The graphic novel may show the slow descent into madness of the protagonists as they uncover the secrets related to Dagon. It might also depict the grotesque and alien landscapes that are characteristic of Lovecraft's writing.
You can try looking for it in specialized comic book stores. They usually have a good selection of graphic novels, including ones based on Lovecraft's works.