fantasy novels comprised of one book

An angel, lost on Earth after failing to return to heaven, reincarnated as a human but retained his celestial powers. As he aged, the power became too much for his mortal body, leaving him lonely despite having siblings. He eventually made a close friend and, knowing his time was short, began distributing his powers through injections of his celestial blood. His friend was the first to receive powers, followed by the angel's sister and many others. To help these new power-bearers, the angel created a separate universe called the Chūkan (The in-between) above Earth with a school in which he called “The Helios Empire academy” where they could learn to harness their celestial aura. The angel ruled the school until his friend, seeking control, demanded a share. When the angel refused, the friend took over during his absence, ruling with an iron fist and sparking a deadly war. The angel returned to find his friend had married his sister, and they had a daughter. Heartbroken, the angel banished them and their followers to a sealed realm called Terra Firma. Before departing on another journey, he appointed twelve powerful beings to govern his universe and placed his niece and her descendants in charge of the school. Then, the angel disappeared, leaving his legacy behind. The in-between (Chūkan) existed peacefully, protected by a vast barrier casted by the Angel to repell all external forces. The students of the Helios Empire Academy were tasked only with suppressing lesser evils within the In-between (Chūkan): vampires, criminals, wicked mages, Dark Elves, stained guardians, and infected shifters. Ten thousand years later, thousands of mysterious meteors fell from the far sky—far beyond the barrier—disrupting it for a single hour. In that moment, the ancient seal placed by the Angels began to weaken, releasing Demons and Devils as old as Chūkan itself. They spread freely across the five continents of the In-between: Gold Gard, Elve Gard, Wizz Gard, Prin.T Gard, and Knight Gard. Now, with the barrier damaged and the continents under siege, it falls to the students of the Helios Empire Academy to protect the In-between from ancient evil