The Outsiders are entities that "live" beyond the Outer Gates. Mortal magic has little or no effect upon them, due to which it takes centuries of study by the oldest wizards on the planet to even slow them down. However, the Knights of the Cross are able to affect them with their power of faith.

The only specific Outsider encountered in the series so far is He Who Walks Behind, whom Harry defeated when he was sixteen, and when he was summoned in Blood Rites. It is debated whether the Lord of Slowest Terror mentioned in Blood Rites is an Outsider.

"While here we wait, O hunter of the shadows! We who cry out in need for thy strength, O Lord of Slowest Terror! May your right arm come to us! Send unto us your captain of destruction! Mastercraftsman of death! Let now our need become the traveler's road, the vessel for He Who Walks Behind!"

Lash/Lasciel had this to say about He Who Walks Behind:

He Who Walks Behind is an Outsider, Harry. A terrible creature, the most potent of the Walkers, a powerful knight among their ruling entities. But when he came for you, you overthrew him.

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