Kuro Kawa to Akunin

The Alphas of the high aristocracy cannot pair with an Omega ($\Omega$) without the King's permission, and Omegas remain under the administration of the kingdom...In this country, there secretly exists a luxurious brothel called "The Academy," where it is possible to check compatibility with an Omega.

At the Academy, to prevent customers from complaining, the director, Knox, gathers distinguished and beautiful Omegas, each fitted with a collar around their neck.In this place, where different stories unfold every night, a proud Omega—who has already been assigned a partner—comes of his own free will... but for what purpose?!