Harry Potter and the Great Old Ones

Jeffrey Rocky stared intently at the lines drawn on the ground, his mind in turmoil. This unprecedented bloodline filled him with fear and unease, yet at the same time, he yearned for the power of immortality.

"Why have I never heard of such mixed-race offspring?" Jeffrey Rocky asked himself in a low voice.

“The Deep Ones have always been a very mysterious race. They have built countless cities in the depths of the ocean, far from human sight and contact. They do not wish to be discovered by the outside world, so their existence has always been a secret,” Tiera explained patiently. “And as I said, their hybrid offspring are not much different from humans. Perhaps your ancestors inadvertently crossed paths with hybrid offspring before they had undergone metamorphosis.”

Tierra used the word "intersection" euphemistically.

In reality, everyone knows what happened: one of his male ancestors slept with a frog-like monster, or more likely, one of his female ancestors was slept with by a frog-like monster.

"So you're thinking," Jeffrey Rocky, now gradually calming down, "that I could act as a diplomat or something similar, facilitating the alliance between the Deep Ones and the wizards?"

“Yes, they are extremely xenophobic and neither believe in nor are willing to accept outsiders.” Tiera nodded.

“I,” Jeffrey Rocky paused, then asked, “Will their immortality necessarily change my appearance?”

“It definitely will,” Tierra affirmed.

"Besides that, are there any other side effects or obstacles?" Jeffrey Rocky asked, taking a deep breath. "You will constantly emit a nauseating fishy smell, your personality will change a bit, becoming somewhat extreme and cruel, and your taste will gradually lean towards raw food."

“But my thoughts and mind,” Jeffrey Rocky asked again.

“You are still you, and that will not change,” Tiera said. A long silence followed.

“If I become a Deep One, will you teach me magic that surpasses all others?” Jeffrey Rocky asked.

"Of course, you are an important diplomat, not only because of your superior magic. When you want to take that step to become a divine being, I will do everything in my power to provide you with free assistance, free materials, free ritual calculations, free setup, and free ritual officiating."

"I...I do." Jeffrey Rocky seemed to have thought about it for a long time, but finally said, "I do!"

Then, as if afraid of changing his mind, Jeffrey Rocky took a step forward, crossing the line that Tyella had drawn with her foot.

“Good, very good, my dear diplomat.” Tierra patted Jeffrey Rocky on the shoulder and laughed. “Of course, you don’t need to worry about the changes in your handsome appearance.”

“While I don’t have the power to reverse Cthulhu’s blessing, I can use some simple spells to keep you in human form without getting wet,” Tiera said.

“Wait, Mr. Tierra, do you mean I can stay the way I am on land?” Jeffrey Rocky asked, his eyes wide.

“Yes,” Tiera replied. “If you are diligent and eager to learn, you will soon be able to transform your form yourself using the flesh magic I have taught you.”