A line in Cold Days grabbed my attention so I wish to figure out what we actually know about them.
1. They exist Outside seemingly all universes, Outside Creation.
2. They are always the Outsiders, and sort-of look different depending on which universe they are trying to get in.
3. They all work together (according to Harry).
4. Outsiders were/are the foot soldiers and servants of the Old Ones (who may or may not be Outsiders themselves), a race of mighty demon-gods that once ruled reality but were locked away (many on Earth, but perhaps some are Outside).
5. Harry accuses them of having alien thoughts and goals - he says the White Council believes they are a crazy rampaging beast, but he believes they are crazy like a fox. Thomas believes that if they work together, they must have a shared motive (which Harry dismisses a bit quickly).
6. They want In. They hate Reality because it is "big and noisy" according to Jim.
7. If they get in, everything stops. Everything. How long that takes is hard to determine (especially considering it was Mother Summer who said it, so time and cause-and-effect is a bit tricky with her due to Intellectus). But it is clearly the end. This seems to be linked to the phrase "Empty Night".
8. They have massive forces attacking the borders of Faerie, that Winter defends and mans; the Outer Gates. Sometimes they barely do much for a few years, sometimes they attack en masse and are very difficult to repel.
9. They haven't broken through altogether - the evidence for that is that Reality still is standing. Although one could make the argument they did manage to get through but were banished again.
10. TWG was the original being who told them to Stay Out, but gave Mortals the power to Invite Them In (i.e. only mortals can summon them in, our only evidence for how this was done was the ritual in White Knight).
11. The power of thresholds around homes and dwellings seems to be linked to the concept that Outsiders must be invited in. It is possible that the rules of hospitality that all supernatural types are big on are also linked to this.
12. The Outsiders have power in a way that normal supernatural types do not - they are "deeper" not necessarily bigger than their inside counterparts.
13. They have a trio of knights called Walkers (although their may be more). No one knows much about them but they are bad news. Initially He Who Walks Behind is the only named one, a powerful Knight amongst the ruling entities (although initially Harry thinks of him as just a bad demon-assassin). We also now have met He Who Walks Before. Behind is as powerful as Uriel, but within certain limits which are not yet know. Before's power is beyond Mab's although he isn't "bigger". There is an implied third Walker who we all call He Who Walks Between/Beside. Behind is "the Right Hand of the Lord of Slowest Terror" who is assumed to be his Old One Lord.
14. Their followers seem to be unusually powerful, especially if tapped into an Old One. The followers of "the Sleeper" were very dangerous, despite being powered up Fomor servitors.
15. Whilst Jim has said there is no real link to the Oblivion War, there does seem to be a connection as Thomas recognized the Holy Ascension of Our Lord Church as being Outsider-worshippers.
16. The Mistfiend at one stage is not an Outsider, but a "mordite-infused gas being from the far reaches of the Nevernever". Later on Harry calls it an Outsider. Curiously, the Far Reaches in DnD is where the Outsider-esque beings come from. And mordite is matter from the Outside.
17. The Shoggoth is the spawn of the Outsiders, but not technically an Outsider per se, which suggests it is the Scion of an Outsider (which could actually be worse - my guess is the spawn of an Outsider and a Mortal). Incredibly powerful, it feeds only on sentient beings getting bigger and stronger as it does. A supernatural doomsday weapon.
18. All Outsiders seem to be very strong, very powerful and very difficult to deal with. Most magic seems to slide off them (supposedly). Harry never has any issues, and neither did Michael's sword.
19. They come from the Netherworld (according to Luccio).
20. The Cornerhound is another type of Outsider, which we know little about. The running theory is that they are the Hounds of Tindalos, and never give up once they gain the scent of their prey. They exist in the angles of time, whereas humans exist in the curves. They are drawn to those who Time Travel.
In Cold Days Sharkface attempts to recruit Harry, saying they always appreciate new talent. Why would he do that? If Outsiders are the originals and are pissed off with Reality existing...why recruit? They just want to bring it down.
The easy answer is that it was a distraction and/or designed to take Dresden off the board so they could succeed. But what if it was serious?
I wonder if Oblivion and Outside are one and the same (I believe that has been posited on here before) and that when all those nasty beings get locked away, they get sent out there. And they wanna come back and are pretty upset. However Jim seems to constantly suggest that the Outsdiers are fundamentally different to anything in Creation, a whole different race. Not like the Fomor, which are a collection of outcasts.
Of course the other reason is Dresden's starborn status (and possibly his Destroyer status). Born with the potential to wield great power over the Outsiders, he seems like a dangerous enemy to them but possibly a powerful leader and ally. The way Morgan talks about Dresden in the microfiction suggests that Harry has the potential to be incredibly dangerous, much more so than he is currently.
Thoughts, theories, suggestions, comments, criticisms, anything I missed?