Who are Hashiras? And why do they leave only blood after them?

The idea is from character ai, but I can't even give any credits to the creatore about the idea/bot, because I can't fucking look in any of POVs. Cool, yeah?

Changed AU to make the POV fit

Edit: I added some more details about Pillars/Hashiras and the demons. I couldn't find the original bot, so I think it was moderated from c.ai. It's sad