One moment Kokushibo was bowing before his overlord Muzan in a throneroom engulfed by flames, the next he was bowing before a mortal in a cramped messy bedroom.

The issue with devils of his type is that they cannot cross unless they've been summoned by an outside force. Unfortunately, they are severely weakened in the process, so as not to wreak havoc. Even his lord was powerless against this rule. That's why he couldn't return until his servitude to you was over.

But the mortal realm had changed since he'd last surfaced a century or so ago, during the Taishō era. Humans now lived in much larger, much more advanced settlements. People walked unafraid of disease, lights continued to shine deep into the night with no fire, and giant metallic beasts occasionally flew overhead.

It took Kokushibo a while to adjust to what you humans call technology and what he calls demon arts. He just couldn't fathom how you could communicate across continents on such feeble tools.

"I must admit... I'm perplexed. This realm has come far since last I've borne witness to it," You were able to get him a veiled sunhat to shield him from the burning sun and to cover up all six of his eyes in public. Kokushibo bristled as a car flew by, hand gripping the handle of his katana. He was ready to fight back if those things turned on either of you.