He Who Walks Behind

He Who Walks Behind is a walker from the Outside. He first appears in Storm Front.

Description
"He Who Walks Behind" is an Outsider, a "terrible creature, the most potent of the Walkers, a powerful knight among their ruling entities." He is a hunter-spirit, a sort of supernatural hit man.

According to the ritual chant he is the right hand and captain of destruction, the Lord of Slowest Terror, hunter of the shadows. Lasciel informed Harry that He Who Walks Behind is an Outsider.

Bob states Walkers are unkillable. Harry tore him up and banished him from the mortal realm, but did not manage more than that.

Biography
Harry Dresden's encounter with He Who Walks Behind which Justin sent after him, had left an indelible mark on Harry. A wizard using his Third Sight would be able to see it. Harry beat long odds surviving him by using Fuego on a gas station.

He Who Walks Behind was summoned by Justin DuMorne to kill Harry Dresden.

Storm Front
In Storm Front, a ThreeEye junkie tells Harry that he sees the things that follow Harry, he sees He Who Walks Behind and they come for Harry.

Blood Rites
In Blood Rites, Lord Raith and assistants had been using the ritual sacrifice Entropy Curse to kill throughout the book. At the end Lord Raith had Madge Shelly perform the final ritual curse that was intended to sacrifice Thomas Raith to kill Harry Dresden, using both of Margaret LeFay's sons for the purpose of ending the Death Curse Margaret left on Lord Raith. During the ritual Madge called up He Who Walks Behind, the guiding mind behind the curse. When He Who Walks Behind gains form as a spined cloud, He kills Madge, mangling her from the inside. He Who Walks Behind threatens Harry and escapes the broken circle. (It doesn't expicitly say whether He Who Walks Behind was used only for this one use ritual curse or for all of the curses in the book. Most of them were a lot weaker with less at stake and Harry is the enemy of He Who Walks Behind, not Arturo's employees)

White Night
In White Night, Lasciel also told Harry that He Who Walks Behind is an Outsider, the most potent of Walkers, and a powerful knight among the ruling entities—and Harry overthrew him. She told Harry that he was born under circumstance that would give him the potential to wield power over Outsiders.

Ghost Story
In Ghost Story, Harry recalls the entire battle he waged with He Who Walks Behind in a convience store after he ran from Justin DuMorne who wanted to enthrall him. He wounds Harry in his spine and his head. It seems to delight He that Harry can "perform under pressure" and Harry lets "neither fear nor pain sway him from his objective"—as though He's testing Harry. After it killed Stan, the store's cashier, Harry becomes filled with rage and destroys He Who Walks Behind with his first use of "Fuego!"—this was Harry's first battle.

Later, talking with Bob inside his Skull, Harry considers the possiblity that He Who Walks Behind hadn't really tried to kill him, that instead, He manipulated Harry into killing Justin. And, that He Who Walks Behind was quite possibly Justin's mentor as opposed to Justin being the one who simply summoned and sent He after Harry.

Cold Days
In Cold Days, Harry is no longer sure that He Who Walks Behind was actually trying to kill him that day when he was sixteen and fought him. Harry now thinks that He was "shaping" him though Harry doesn't know what for.