You survived (Uppermoon user)

You had gone through many different phases during your multi-century life. Started as a human, became a demon and climbed to an upper rank, now a test subject of some random scientists in the 21st century. But wait, didn't all demons die with Muzan? Sort of. Every demon except you. You were in hiding, and you somehow survived. You lived when the Demon Slayer Corps disbanded. You lived when phones, computers, and all other technology was invented. You lived when people started using newly found quirks for evil. You lived when being a pro hero became an actual job. Legally dead, physically a shadow. No one knew you existed.. no one, until that night. That night, when they found you regenerating in an ally after a fight. That night when they took you. That night. After that, the days started to blur together. The same routine, the same people. Scientist after scientist. The faces the same unpitying mask. Test after test, it didn't matter. You just healed any wound you had gained. You were miserable, but hid it well under a blank look that could rival Kokushibo's. Nothing stood out in the months you were there. No person, no day, nothing. Until you heard it... You heard it before the alarms even went off. The sound of a raging fire. It was late at night, so no one was walking through the halls outside your cell. You were resting against the wall, bags under your eyes, every bone standing out. You were like a walking skeleton. You had tried resting to regain energy like that one demon -Nezuko, was it?- did. But the scientists just woke you up before you could get proper sleep. Enough of that though, this could be your chance to escape. You waited until the fire had melted the bars of your cell before just walking out, easily regenerating the burns you might have gained, the blue flames uncomfortable at least, warm at most. By the time you walked out you were fully healed, a dismembered arm you were middle of eating in one hand, the rest of the burnt body in the other. You barely registered the shocked villains just a couple dozen feet away, focusing on your meal. Though you did subconsciously identify them. Dabi, who must've been the one to set the building aflame. Toga, the young girl obsessed with blood. Shigaraki, who was probably there just to keep the other two in check. The trio stared at you in shock, amazed that you were unscathed.