Prologue:
Long, long ago, in the times of swordplay and magic. There once was a powerful deity, recognized for it's cold-blooded nature, leaving death and despair in it's wake. This being was known as Grima.
Grima was an extremely powerful and sinister monster, enemy to the Goddess, Naga, as it's dark magic and terror spread across the land. However, there was a hero who stood tall and managed to defeat Grima, bringing peace and prosperity to all. This man was titled as the Hero King, Marth. Marth charged together against the mighty being with his army, slaying the dragon with the legendary sword, Falchion. Thousands of years later, Grima now lies down in a deep slumber, patiently waiting for someone to revive him
"A being of pure darkness and despair? Sounds like my kind of guy! Let's see if I can't pay him Ra little visit, might even make a deal with him for fun!" The being, Bill Cipher, had been looking through different dimensions. He had been growing bored of the Pines family, as there was no conflict going on the he could take advantage of, at the moment. He began chanting a strange ritual, as he was teleported into a room, filled with pure darkness and no light to be seen.
"Sheesh, I know he's supposed to be all evil and everything, but the least he could do is make his chamber of eternal slumber look a little classier."
The demon then floated around for a bit, traversing through the darkness, eventually hearing a loud growl from behind him. The triangle turned around as he was now face to face with an eighty foot, black and purple dragon. Steam emitted from its jaw as it spoke with a demonic and dark voice.
"Who dares disturb my slumber?"
The dragon heard a light chuckle, as the triangle floated up to his line of sight.
"That would be me, ol Grimmy Gram. Name's Bill Cipher, and I've got an offer you can't refuse." The demon spoke, his eye flashing blue for a split second.
The Dragon looked at the demon, slightly curious. Grima could sense the raw energy emitting from the being, intrigued at the power of this new-found creature.
"You intrigue me, demon. Despite your small appearance, you clearly hold much power. What is this offer you speak of?"
The demon laughed, placing his arms behind his back.
"Well, that's simple. We both have something in common, we want to spread darkness and have the world beneath our heels. I can help wake you up from this whole slumber thing so you can begin your reign of terror again, and all I want is to borrow a bit of your power for a little so I can grab you the vessels you need."
The demon's eye glew as it showed a hologram of two twins. The first twin was showed as a bright and bubbly young girl. The other twin was showed as a slightly anxious and wary young boy. The dragon stared in confusion, showing slight doubt in its glowing red eyes.
"Children? You're going to have children as my vessels?"
The demon dissipated the illusion as it looked back at the dragon.
"Yep, these two would make the perfect vessels. You should know that kids are easy to trick. Getting them them here would be a piece of cake!" The demon said. Twirling his cane.
Grima contemplated his thoughts, eventually coming to a conclusion.
"Very well, I'll accept your offer. I've slept for far too long, it's about time I've returned to this world.
Bill laughed, making his cane vanish as he held out his hand, engulfed in a blue flame. "Good choice, You've got yourself a deal!"
The dragon temporarily took a human shape as he shook hands with Bill. Bill's eye glew a bright blue again, vanishing out of existence with a mad cackle, setting his sights on the unaware duo of Gravity Falls.