You didn't make this world. You came back to it.
Every world has a first player. In yours, something went wrong with theirs, and the game is still trying to bring them back. It's using you as the template.
Nothing happens on the first day. That's on purpose.
It stands at the edge of your render distance, facing you. Look away and it's gone.
Or it's closer.
It remembers what you say, where you sleep and how you walk. Your torches won't stay where you put them. Your footsteps will sometimes take one step more than you did.
Its body doesn't move while you watch it. Its head is another matter.
When it finally comes for you, it's faster than you can sprint, but only while you aren't looking at it. Looking at it has a cost too.
The game itself stops being trustworthy. Menus, messages and screens you've seen a thousand times start telling you things that aren't true. Dying doesn't reset it. Neither does making a new world.
The Reborne only writes inside your .minecraft folder (its own reborne/ folder, screenshots and crash-report text files). It never deletes saves, never reads your personal files and never touches your OS. If it's ever too much, delete .minecraft/reborne/ or run /reborne reset.
⚠ Photosensitivity: it uses screen static and flicker, plus low-frequency sound. Please play with care.
<details>
<summary>For testers and content creators (spoilers)</summary>
/reborne phase <0-4> skip ahead (-1 = automatic) /reborne spawn <stalk|creep|hunt|watch> /reborne event <name> /reborne reset
Requires cheats. It ignores creative players.
</details>
If the window title says something it shouldn't, that's not a bug.