So in Peace Talks, Harry banishes the Cornerhounds calling them "Servants of the Outer Night". Where have we heard part of that phrase before? Oh yes, the Lords of the Outer Night.
There are 13 Cornerhounds. Eb calls them "the whole pack". He also explains they are not actually 13 separate beings, but the one entity. Harry explains this as the entity is not actually in the mortal world, it creates a drone or avatar (or several) in which so visit the mortal world while safe in it's home reality.
Interestingly, there are also 13 Lords of the Outer Night. However, the Red King "Kukulcan" is an anomaly. If the Lords of the Outer Night were like the Cornerhounds, wouldn't they be limited to 13 as well? And the Red Court leave skeletons, not ectoplasm (except for their flesh masks I believe). Which would suggest they are more "real".
My theory is that the original Entity is Kukulcan, the Red King. He possesses a mortal human, perhaps even a starborn, perhaps not. But the being is so big that it cannot be contained in a little human shell. This is also part of why the vampires look human much of the time, likely the other reason is to blend in. When the Entity spreads the majority of it's power out amongst it's "children" they become the 13. Not a gestalt being as such, separate minds. Almost a combination of the original entity plus the mortal it inhabits. Yet intrinsically linked to the greater entity. The downside of course of being in the mortal world is it makes them more mortal...whereas a "drone" while destructible represents no great loss to the Entity as it is more like a tool than anything else. Perhaps it is the combination of mortal and immortal, sprit being and physical, that allows it to act. It might not be able to act anywhere near as much without a mortal, physical body. It's not like Harry or anyone else considered ever banishing the Lords of the Outer Night.
I suspect this is the case with the other Vampire Courts, and is what links them. It's also why they must feed on mortal blood/life. They have none of their own and cannot produce it, and therefore cannot maintain that level of being "real" without it. '
Perhaps Drakul is similar to Kukulcan, but undoubtedly worse. Curious he doesn't have 13 servants (although perhaps he used to - Jim said he always liked an elite team rather than a full Court). The extension of the power into a Court seems to weaken them in some way. I think it is in Changes that they discuss how the power of the blood of the Red Court is now spread amongst too many beings. In trying to grow larger there is less for all to take perhaps. Drakul seemed to want to avoid it, and when his son Dracula expanded things to a full blown Court, that's when the Black Court was nearly destroyed. Perhaps the trade off is between the weight of numbers versus individual strength. I imagine the White Court and Jade Court will be rather similar, and perhaps had similar beings as their progenitors (although likely they were weaker, I imagine it goes Black, Red, Jade, White in terms of strength).
But the salient point is there is a clear link between the Outsiders and the Red Court, and quite a lot of less obvious links between the other Courts and the Outsiders. Curious also that the Red Court Elders are styled "Lords" of the Outer Night. I never felt them on the scale of any of the most powerful beings of the Outside.
Also, I would be willing to bet that Nemesis (He Who Walks Beside) can only possess 13 beings at once. Also, I think the Walkers all must have possessed a mortal. He Who Walks Behind possessed Madge's body in White Night. I never saw anything about any of the Walkers turning into ectoplasm after death. They leave corpses of a sort. Which must mean they have a mortal, physical shell (even if the being itself is immortal). Nemesis always seems to be in a real body, rather than a created one - in fact so far we haven't seen him in his "true" form like his counterparts.