Outsider

The Outsiders are very powerful beings from the Outside, which is beyond the Outer Gates. Even though Walkers appear earlier in the series, they are first mentioned in Death Masks.

Description
Little is known about them. The Outsiders had been the servants and foot soldiers of the Old Ones.

Outsiders are immune to most forms of magic. It's hard to make them leave, it's hard to hurt them, or kill them. "They are insanely violent, insanely powerful, and just plain insane."

The White Council's Seventh Law of Magic forbids the contacting of Outsiders.

Outsiders cannot enter our reality, without someone letting them in through the Outer Gates. Only mortal magic can call up Outsiders—it wasn't the Vampires that called them up in the battle on the Ways.

They collaborate very effectively, making them most dangerous.

They are different from creatures from the Nevernever, such as Demons, in that Demons are a part of "our world"—Outsiders are not, they are from Outside.

The White Council thinks that Outsiders are crazy creatures that go rampaging in any random direction. Harry disagrees saying they are "crazy like a fox." They are good at psychic assaults and they love using them.

Notable Outsiders are the Walkers and include He Who Walks Behind and He Who Walks Before.

In Cold Days, three Walkers are mentioned, He Who Walks Before, He Who Walks Behind and one still unnamed.

Death Masks
In Death Masks, for the test of Will at the Duel between Harry Dresden and Duke Paolo Ortega, The Archive chose to use a sphere of Mordite. It looked like a soap bubble with tentacles. It is called Deathstone because it kills anything living that it touches. It is not of our reality. It's "congealed antilife" from the Outside.

Dead Beat
In Dead Beat, during The War, the Red Court had called up Outsiders against the White Council, who were retreating with wounded, along the Ways in Faerie. Ebenezar McCoy and Martha Liberty held the Reds off them long enough for Arthur Langtry and the Gatekeeper raised a Ward to block them while the Wardens escaped with the wounded.

Proven Guilty
In Proven Guilty, Michael Carpenter was with the Wardens at Luccio's training camp in Colorado when he got Harry's message to come right away with Council Members. So, he happened to be with Luccio, Ebenezar McCoy, Listens-to-Wind and the Wardens when they traveled through the Ways and fought Outsiders with fangs and tentacles.

White Night
In White Night, Lasciel told Harry that Vittorio Malvora was possessed by an Outsider and given power that enabled him the immense ability to psychically attack which he was right then, wielding in the Battle of the Deeps, crippling everyone present. 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 circumstances that would give him the potential to wield power over Outsiders.

Turn Coat
In Turn Coat, Samuel Peabody released a Mistfiend at Donald Morgan's trial to cover his escape. It killed several wizards and it took the entire Senior Council to contain it.

Cold Days
In Cold Days, the Outsider Sharkface attacks McAnally's Pub when Harry refused to give himself up. When discussing Sharkface (He Who Walks Before), Harry told Thomas what little he knew about them. Mac let Harry know there are three of them, by symbolically using three beer bottles to cover for giving Harry information. Someone is letting in Outsiders in a plot to release what was imprisoned underneath Demonreach by The Original Merlin: six Naagloshii and hundreds of dark gods.

Dresden and Murphy leading the Wild Hunt, including the Erlking and Kringle, battle Outsiders led by Sharkface on the shores of Demonreach.