Gunzougeki no Crown ~Crown Never Die, They Just Fate Away~

One hundred years of war brought the world to the brink of collapse.
A hero managed to stop the apocalypse. 40 years later...

The year is 2097. During the war advances in magic and science permeated all things.
Humans were bred with special powers. No longer needed in these peaceful times, these "specials"
were herded into a closed city. One young man was born here. His name was Misaki Amemiya.
Like others he spent his days uneventfully, each one identical to the last...
until everything suddenly changed, and a mysterious woman wielded a sword on Misaki.
He prepared for death. Just then, a young girl appeared.
She had saved Misaki's once before.

She said these words:
"I will protect you."
Then she went to battle with the woman.

[From DLsite English]