Underspell is in AU in which the Sans of this universe becomes a Protector of Other AUs.
Long ago, there were two races that ruled the Earth: Humans and Monsters. One day, war broke out between the two races. The Monsters and Humans were equally experienced in magic, but humans were victorious after conducting darker rituals. A number of Human witches cursed the monsters, making them turn to dust upon death. This is what allowed the Humans to win. The humans then sealed the Monsters underground with a magic spell that could only be broken with a power equivalent to 7 human SOULS.
In the Underground, the Monsters, known for their magical abilities could not only send harmful projectiles out of thin air, but could also control for of the basic elements: Air, Earth, Fire, Water. Some practiced alchemy, others practiced voodoo. Some dealt in healing spells, and a select few practiced the dark arts, or Black Magic. Being in the underground, there was no way of telling the time of day, as there was no sun nor moon, so spells and rituals involving either of the two were not an option. Some spells were limited. In the Ruins, there we those who had terrakinesis, or "Earthbending". Toriel knew Firebending. In the Snowdin Forest, Snowdin Town, and Waterfall, there were the Waterbenders. Hotland had the Firebenders. New Home was home to a few Airbenders.
One day, a skeleton known as Sans had a vision. A human, with the power to destroy EVERYTHING would come into the Underground and unleash havoc. In his vision, he saw monsters turning into dust, cities being empty, and a horrifyingly creepy smile and blood-red eyes. Sans sought to learn all kinds of magic. He went to the Ruins for knowledge in Earthbendng, Hotland for Firebending, and New Home for Airbending. Sans already knew Waterbending as he and his brother grew up in a Waterbending village.
Once Sans had the knowledge of those powers, he started training/studying in other fields of magic. Many years later, in the year 2010, as Sans had just finished learning and practicing Healing Spells, he heard of a human who fell down in the Underground. This was Chara (pronounced chara). Chara was taken in as a child to the Royal Dreemurr family.
Eventually, Chara and Asriel both died and the King declared war on the Humans. Sans then decided that he should practice Alchemy. 6 more humans came in the following six years, one human each year. Then, in 2017, an 8th human fell into the Underground. Sans was still only learning the intermediates of alchemy. He felt that this would have to suffice. He knew that this was the human from his visions. As he exited the Ruins, Sans greeted the Human. To his surprise, the human was a child, who could not have been older than 11. There was dust on the child's clothing, but something told Sans that this was not the stone-cold killer from the visions.
The human made their way through the Underground. The child had killed some monsters, even gaining a Level of Violence of 11. But when Sans judged the child, he knew the child did not want to harm anyone and was only acting in self-defense. The child had spared monsters in their travels, but most important to Sans was that the child had spared Sans's younger brother Papyrus. They eventually freed all of the Monsters. Sans was happy and confused at the same exact time. On one hand, everyone was free. On the other, Sans was so certain that he found the killer. In Sans's vision, the human was a killer. It looked exactly like the human from his visions, yet the human turned into a pacifist. Sans then thought that he saw another possible reality. Sans then thought of exploring the realm of science.
Eventually, Sans was visited by an odd looking version of himself. The figure had black bones and looked... broken from his reality, constantly shifting and pieces of him coming off and rejoining him repeatedly. Sans then found yet another version of himself. This Sans was wearing a yellow jacker, brown boots, orange stripes on his pant legs, and carried an axe. His eyes were also two different yellow shapes. The Sans of this universe then helped the person in yellow. They both drove off the night-boned attacker. The skeleton in yellow revealed himself to be "Slayer Sans", a Protector of Alternate Universes, or AUs for short. He then said that the attacker was known as "Error Sans". Error Sans was an AU destroyer, the most prominent and powerful. Slayer then asked the Sans in front of him if he wanted to join. The Sans of this universe agreed. Slayer Sans then dubbed the new recruit, "Magic Sans".