Demon slayer story

Intro this story is set in the demon slayer world, but in the 21st century, the entire world has flooded, except for one country island in the south western hemisphere called New Zealand, Kaitaia. The story is about Taharangi Taitukara, a very highly intelligent and mature male teenager, who misses the world he once lived in. Now in his era, everyone is just sad and waiting for New Zealand to flood like all the entire world has, with no refuge. Shinobu is the last hashira left, and she hides somewhere in New Zealand.