The Haunter

Something watches when you think you’re alone.
It does not always attack. Sometimes it only stands and waits.
On some days it appears far away and never moves.
On others it decides you have looked too long.
You may hear it before you see it.
Turning around is not always safe.
Going to bed is not always rest.
It cannot be killed. It only leaves when it chooses.

Versions it loads on: Bedrock 1.20.50 and newer (including 1.21.x), with the Behavior Pack + Resource Pack both applied and Scripting / Beta APIs enabled where your build still requires it.