Depending on the specific community you found this in, it likely refers to one of the following:
A psychological drama directed by Jaime Rosales. While not "horror" in the traditional sense, it deals with dark themes of family secrets and ruthless characters, which often triggers "creepypasta" or mysterious edits online. petra.mov
A creator named @faceoffpetra has been posting surreal, unsettling content including videos about a "scary man next door" and original horror-themed music. Depending on the specific community you found this
The "petra.mov" file appears to be part of a niche or Analog Horror ARG (Alternate Reality Game). Based on current digital traces, here is the report on its context and significance: 📂 File Analysis: petra.mov petra.mov