Little is known about them. The Outsiders had been the servants and foot soldiers of the Old Ones. Outsiders cannot enter our reality, without someone letting them in through the Outer Gates. Only mortal magic can call up Outsiders; the White Council's Seventh Law of Magic forbids contacting them.
Outsiders are immune to most forms of magic. It's hard to scare them, to hurt them, or kill them. "They are insanely violent, insanely powerful, and just plain insane." 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 Dresden disagrees saying they are "crazy like a fox."  They are good at psychic assaults and they love using them.
In the seriesEdit
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.
In Blood Rites, Lord Raith and his assistants have been enacting the rituals required to produce the entropy curses. At the conclusion, Lord Raith had Madge Shelly perform the final ritual 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 cast on Lord Raith. In the ritual, Madge calls up He Who Walks Behind, the guiding mind behind the curse, which gains form as a spined cloud, then kills Madge, mangling her from the inside. The Outsider then threatens Dresden and escapes the broken circle.
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.
In Proven Guilty, Michael Carpenter was with the Wardens at Luccio's training camp in Colorado when he got Harry Dresden's message to come right away with Council Members. So, he happened to be with Luccio, Ebenezar McCoy, Joseph Listens-to-Wind and the Wardens when they traveled through the Ways and fought Outsiders with fangs and tentacles.
In White Night, Lasciel told Harry Dresden 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 Dresden that He Who Walks Behind is an Outsider, the most potent of Walkers, and a powerful knight among the ruling entities—and Dresden overthrew him. She told Dresden that he was born under circumstances that would give him the potential to wield power over Outsiders.
In Cold Days, the Outsider He Who Walks Before attacks McAnally's Pub when Harry Dresden refused to give himself up. When discussing Sharkface (He Who Walks Before), Dresden told Thomas what little he knew about them. Mac let Dresden know there are three of them, by symbolically using three beer bottles to cover for giving Dresden 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.