Little of the Outsiders or “Walkers” has been revealed. They may be the most powerful of the Demons, or stranger and even more powerful entities akin to Lovecraftian Great Old Ones. It takes wizards centuries to develop defenses against them and magic apparently has little effect upon them. They can be defeated by, among others, the powers of the Knights of the Cross (Michael faced and beat several of them near the end of ‘Proven Guilty’); Harry Dresden himself also faced and defeated a particularly infamous Outsider known as “He Who Walks Behind” at the age of sixteen, as part of the final confrontation with his mentor Justin DuMorne.
However, according to Bob, Outsiders cannot actually be killed, only damaged sufficiently as to require long years to recover; this was confirmed in ‘Blood Rites’ when He Who Walks Behind was summoned by a mortal magician and recognized Harry before escaping from the disrupted summoning ritual, promising to wreak vengeance. As Harry claims only a sketchy memory of the fight and sustained many scars both physical and metaphysical from the struggle, this impending confrontation may be the most dangerous of Harry’s life. The fallen angel, Lasciel, has told Harry that he may possess a special power due to the timing of his birth, that could potentially give him the power to actually kill Outsiders, a deed believed to be impossible at this point.
Wizards are terrified of the Outsiders and of their power; however, as only mortal magic can summon an Outsider (neither vampires nor faeries can do it), the temptation to gain power can be irresistible, and the Seventh Law against Outsider summoning is unique among the Laws in that a single Senior Council member is assigned to enforce it full-time. (Though the Blackstaff of the Council, as part of his general exemption, may summon an Outsider it is yet unknown whether one has ever done so.) Their internal organization is only sketchily understood, but they appear to have a caste system which includes empires and knights.
It is also known that Outsiders can possess the mind of a White Court vampire. Before his death, Vittorio Malvora had performed massive amounts of magic and put several people into submission. Lasciel also told Harry that Vittorio had been possessed with an Outsider, hence gaining the ability to do this. It is also hinted at in ‘Blood Rites’ that Lord Raith might have some protection granted by the Outsiders to protect him from mortal magic.