Seraph of the End: Guren Ichinose: Catastrophe at Sixteen (manga)

When the world has only days left, what do you cling to?

After becoming the head of the Ichinose clan and acquiring even more powerful cursed gear, Guren seems to have finally caught up to his childhood love Mahiru. The achievement is short-lived, however, as he soon learns that Mahiru has taken yet another impossibly large leap ahead of him. She leaves a bloodied and bewildered Guren with a mysterious piece of cursed gear to give to Shinoa, her little sister, then shortly after marches on the Imperial Demons with a faction of their own soldiers to initiate a vicious insurrection. 

But with only two days to go until the apocalypse and no idea how to stop it, Guren finds himself forced to ask the inscrutable ­Mahiru for help. The final countdown has begun, and there is no turning back. All that remains is the promise of catastrophe—and one last chance to save the world.