Unfallen (sanegiyuu/ sanemi x giyuu)

Humanity is broken. Cities lie in ash, and the night belongs to monsters.

Decades ago, a secret experiment sought to create the perfect human - but it failed. Three nearly-perfect subjects escaped, unleashing a plague of creatures upon the world. Most are mindless beasts, but some inherited their creators' cunning... and hunger.

will samemi and giyuu survive?

Author's Note:
my au
not cannon I modified the demons, and demons are called something else - for more info see "Demons"
In this au there is no breathing styles , but i compensated it with something else- for more info see "slayers"