In the fanfic, it might be interesting if the illithid comes from a different dimension entirely and brings with it a new type of magic. Harry Potter, being the curious and brave character he is, decides to learn this new magic from the illithid. However, this new magic comes with a price, and Harry has to make difficult choices about whether to continue learning it or not, all while trying to keep Hogwarts and the wizarding world safe from other threats.
Obviously, the illithid would be a main character. It could be a mysterious and powerful being that Harry encounters. Also, Hermione and Ron might still be important characters. They could be either suspicious of the illithid or try to help Harry understand it better. Dumbledore might also play a role, perhaps guiding Harry in his interactions with this strange new creature.
The main distinction of porn with a story genre from regular porn is the presence of a coherent narrative. Regular porn often jumps straight to the sexual action, while the story genre tries to add some kind of story framework around it. But in the end, both are adult-oriented and feature explicit sexual content.
Well, it's hard to say for sure. The concept of 'porn-like' is quite subjective. Some might consider certain novels with excessive sexual descriptions as such, but it depends on individual perspectives and cultural standards.
In many places, 18+ porn comics are considered illegal. This is because they can have a negative impact on society and violate moral and legal standards.