The Slayer From The Past

What if the TRASH OF THE COUNT FAMILY event and DEMON SLAYER event happen in the same world but the TOTCF event happen after 400 years and DEMON SLAYER event happen 400 years ago? Well this is a story of a slayer that have been alive for 400 years and now helping the mc of the story saving the world. How the slayer can be alive even after 400 years? Well turns into a half demon have it pro after all. Now let's see how the existence of this slayer can change the future

Ps: this is a new description since the original one is disappear which I don't know how that happened. Also credit to whoever own the photo and video I'll be using and THIS IS MY AU SO DONT TELL ME THE DIALOGUE IS NOT THE SAME BECAUSE I'LL CHANGE IT TO FIT THE STORY and because I didn't really remember the plot and storyline of the manhwa and anime