MC lives in a world where humans were in a 4 way war with mythological races but won in the end. MC and friends are guards for some sacred site where the four races where said to be sealed. MC is super serious about his job despite nothing happening for centuries.

One day, something happens that tears the fabric of reality and he wakes up in a destroyed city. To his surprise, demons show up, terrorizing the population. He fights them and later learns he's in a world where humanity lost and the four races took over. Another thing, his friends and everyone he knew is there but apparently he doesn't exist in this world.

I'm no angel, but I'm more pure than you.

Hi there.

I'm quite certain this is Why Does Nobody Remember Me in This World?

An anime adaptaion for it just got announced.

Hope this helps.

Yes, this is it. Thank you~

I'm no angel, but I'm more pure than you.