The Nova Slaying Shinigami

Seishin Samayou, awoken from a 20 Year coma, is back on his feet and with new powers. He is then taken to a school where genetically modified girls called Pandora fight against entities called the Nova. Will he master his new powers and save the world while being the only male of their kind? (OC x Harem(Akame Ga Kill, H.O.T.D. and Highschool DxD included)(MORE LEMONS COMING UP!)