The Serpent Lord's Spelled Mate

There's a myth in the human world that when a snake turns 100 years old, he/she can shapeshift into a human form. In other words, they become half-human. There have been countless works of fiction based on this myth but that's all. To humans, they're works of fiction. No one ever has been able to discover that under this human world lies the splendid world of snakes, the Serpent Realm. There are snakes who can shapeshift into humans and choose them as their partners. The choice is always theirs. It was all going well until an exception occurred. When the next in line Serpent Lord shapeshifted twenty years before his coming of age ceremony, he found that there's a tattoo-like mark on his human body. It wasn't an ambiguous mark, rather a very clear one, indicating towards something very specific. It occupied his right arm between the shoulder to the elbow. It looked like a tattoo and with the help of the eldest priest of the Serpent Realm, the Serpent Lord came across a shocking revelation. It was a mate mark. A mate mark, that has never appeared on any snake because they're free to mate with whomever they want, appeared on the serpent lord's arm. Along with the mate mark, they happened to come across a million years old prophecy. This prophecy was enough to force the Serpent lord to leave his Realm, go to the human world, and wait for twenty years before he could meet the girl who was an integral part of the prophecy.