Sōma

This demon has no solid body, appearing instead as a ghostly deer skull ringed by a blue glow.

The creature drains the memories of anyone it makes contact with. It can just as easily seize control of a person, twisting their personality to serve its wishes and routing its power through that host. Because it exists only as a spirit, it stays intangible, and most strikes pass straight through, though sealing techniques can still harm it.

Four hundred years back, Sōma comes close to wiping out the Daidai Village, whose people wither under its presence, until a shrine maiden by the name of Kuen stands against it and locks the demon away inside her own body. Yet as her memories bleed out too fast, she cannot cut off both of the sprouting horns that would finish the binding, so the demon lodges instead in a rock nearby while a sliver of it remains within her. Kuen loses her memories permanently, and the same curse also threads down her bloodline. Once any two who carry it turn twenty, a mark rises on their palms to mark them as Sōma's tie to the living world, and their memories begin to seep away. Each passing century the binding loosens, letting the horns push out from the rock as the demon regains strength, and the only way to lock it back down is for a cursed man and woman to marry and renew the seal.

When the seal weakens yet again, Sōma baits Kankitsu Akitsuki, betrothed to Remon Yoimura, into its chamber and takes him over, then uses that puppet to shatter the binding and get loose. Boruto Uzumaki and Konohamaru Sarutobi arrive to stop its rampage, and though they cannot bring it down, Remon calls on the very seal her ancestor Kuen once wielded to bind the demon inside herself. With Konohamaru's grudging help the rising horns are cut through, finishing the seal and caging Sōma for good, a triumph that costs Remon every memory she carried beforehand.