The 1987 Tulsa Missing Boy is an documented Slender Man-related incident, from the Original Mythos.
Newspaper[]
The Oklahoman
August 21st, 1987
New Discovery In Disappearance of Boy
Tulsa, OK— A new piece of evidence has turned up in the investigation of the boy that disappeared from an elementary school in Tulsa. The boy's father, it was discovered, was filming his son's first day of school, and the home video footage was released to local authorities. He took his own life shortly after the boy's disappearance. No word yet on any suspects, though police are drawing similarities to the 1963 slayings in Lake Texoma. The voice on the tape has yet to be confirmed and though the film's video is largely destroyed, the audio remained intact.