Adopted by the Demon Lord

The story of a human from one world summoned to another world is familiar and reliable, but when the hero gains a reincarnation skill that allows him to come back through the lives of his descendants he gets bored of living forever. The only one who knows of this skill besides himself is the Demon Lord Saiaku Tou, whom he became friends with during one of his many lives. Throughout those lives the hero becomes sick of living and invites Tou to finally kill every human including the hero himself. Though he asked one favor in return, "If you can find one human, just one, that is redeemable. Promise me that that one human can go on living in your care." Tou took him up on the offer, but when the humans were all dragged back to the last human city standing he finally found the human he was looking for, a little girl who suffered even though she had committed no sins.

(Outside of the beginning prologue and ending epilogue, each chapter will be split into two parts. One for the pov of the demon lord, and one for the pov of the little girl.)