Owari no Seraph: Ichinose Guren, 16-sai no Catastrophe

Summary

"Hey, Guren? Do you think... when we grow up, we can still be together like always?" Just before the world fell to ruin because of a mysterious virus that appeared on Earth which killed every infected human over the age of 13. Vampires conquered the surface-in that one last spring. Ichinose Guren had entered a Sorcery Training School in Shibuya. This school was full of elites from famous families in the world of sorcerers. As he himself was of modest origin, Guren held a great ambition in his heart, even despite being labeled as trash. But among his classmates, a girl appeared who introduced herself by the name of the fiancée he had traded vows with a really long time ago... In that world slated for ruin, the boy sought power; and the girl sought power as well. A magic academy fantasy story begins! (Source: owarinoseraph.tumblr, edited)