A demonic entity from the Outside known as a Walker in the Dresden Files novels. He can be summoned only by mortal magic and was sent after Harry Dresden by his guardian, Justin DuMorne, when he resisted an enthrallment spell.
Harry was able to defeat He Who Walks Behind, thanks to the special timing of his birth, which supposedly gives him the power to kill Outsiders. This was not without injury however, and he bares a scar on his soul that can be seen by those with the Sight.
He is also one of the many entities that can sponsor power for a ritual, and was used for a particularly powerful entropy curse against Arturo Genosa's associates in Blood Rites.
"You who is marked by He Who Walks Behind!"