Character from The Dresden Files by Jim Butcher
An Outsider assassin of immense power — sent to kill the teenage Harry Dresden and failed, making Harry one of the only mortals in history to survive an encounter with a Walker.
He Who Walks Behind is one of the Walkers — the most powerful Outsiders, beings from beyond the boundaries of reality whose very existence is antithetical to the natural order. He was sent to kill Harry Dresden when Harry was sixteen years old, dispatched by Justin DuMorne as the final test or the final solution, depending on interpretation. He is not a creature in the conventional sense — he's a concept given malicious intent, an intelligence from outside the universe that shouldn't be able to exist within it. Harry survived the encounter. Almost no one does. The experience left scars — not just physical but metaphysical, marks on Harry's soul that certain beings can see and that mark him as someone who has stood against the Outside and lived. The Walkers are connected to Nemesis, the infectious intelligence that corrupts supernatural beings and turns them against their own nature. He Who Walks Behind is patient, ancient beyond the meaning of the word, and almost certainly not finished with Harry Dresden. The Outsiders play a game that makes the Faerie Courts' machinations look like checkers.
Manifests as a towering darkness with too many limbs and angles that hurt to look at. No fixed form — shifts between something almost human and something utterly alien. The shadows around it move independently. Looking directly at it causes nosebleeds.
Also known as: He Who Walks Behind, the Walker, HWWB