Ten'yoku no Asklein NIGHTMARE CURSE

A nightmare that could have happened— a curse suddenly fell upon them.

Asklein Crysta and Sukuna no Himemiko continue their fight to protect people from the evil deeds of the Descending Demons.

One night, they come face to face with a demon wielding sorcery. The two manage to repel the demon, but the demon uses its "magic" as it dies.
The next thing the two realize, Crysta has been captured by researchers and is about to be used as an experimental subject, and Sukuna is about to be used as a plaything for the demons...

[From DLsite]