Long ago, a powerful demon named Soma almost destroyed a village called Daidai. A brave shrine maiden named Kuen trapped the demon inside her body but couldn't fully seal it. Instead, she locked it in a nearby rock, keeping a part of the demon within her.

Because of the demon's curse, Kuen lost her memories, and the curse was passed down to her descendants.

When two cursed people turn twenty, they get a mark on their hands, showing their connection to the demon and their memory loss. Every hundred years, the seal weakens, and the demon grows stronger with its horns growing from the rock. To prevent the demon from escaping, a man and a woman with the curse must marry and strengthen the seal again.

As the seal weakened, Soma tricked Kankitsu Akitsuki, who was engaged to Remon Yoimura and took control of him. With Kankitsu's help, Soma broke free from the seal and escaped. However, Konohamaru Sarutobi and Boruto Uzumaki appeared to stop Sōma's rampage. Even though they couldn't defeat Soma, Remon used the ancient seal of her ancestor, Kuen, to trap Soma inside her.

With some hesitation from Konohamaru, they managed to cut off Soma's growing horns, completing the seal and permanently trapping Soma.

Soma was a cunning and selfish demon who wanted to live on other memories, which is why he tricked Kankitsu Akitsuki.

Soma is a demon spirit that appears like a deer skull with blue color aura around it.

Soma can steal memories and live on to that. It can control other people and is also very powerful, as it breaks through many sealing jutsu. It can also shoot some of his spirit bombs.