What do all these things have in common?
Hellboy.
Let me tie it all up.
HELLBOY
In Hellboy, the Watchers are the Angels and the first of them "Anum" creates the Ogdru Hem and the Ogdru Jahad. OJ are the Dragons of Revelation and the sires of the OH. The Watchers and the Ogdru Hem and Jahad go to war, and the Watchers are victorious. The Watchers then kill almost all the OH and seal away the OJ in the Abyss (not quite Hell, but adjacent and full of weird tentacle things and borders Chaos). They then strike down Anum out of fear of what he had done, leaving only his Red Right Hand. The Ogdru Jahad are essentially based on the Lovecraftian Old Ones, and so horrible to look it they often induce madness.
The other lesser Watchers become the Hyperboreans, the first golden race of man (in pre-history). They found a great empire and learn the secrets of their forebears, places that include Atlantis and are led by a great king called Thoth. Thoth has three Watchers captured in his garden and learns all the secrets of the universe, inscribing them in 42 tomes. He gives two to his people and keeps the rest for himself. He is then seduced by a woman called Hecate, who he teaches much to. She then kills the Watchers and drinks their blood, vomiting it back on the walls of the garden and learning terrible secrets and becoming a major power. She then shares the information with the Hyperboreans that Thoth kept from then and says do what you will. The Hyperboreans split into two factions - the People of the Right Hand who worship Thoth, and the People of the Left Hand who follow Hecate, the Black Goddess.
The only piece left of Anum was his Red Right Hand (also called the Right Hand of Doom, as it alone could free the OJ). This eventually falls to Hellboy whose true father cut off his right hand and replaced it with the Right Hand of Doom.
The Frogmen in Hellboy are the final race, intended to inherit the Earth. They are both powerful physically and psychically, they use mind tricks and illusions and can switch between mortal and human form and they also have a poisonous saliva. They are linked with the OJ and the OH and are raising them so they may rule the world once again. They also are linked to powerful corporation called Zinco.
There is a man on an island, who is the repository for the history of the world. His name is Mhlomi.
Pluto (Hades) was the first rebel angel and the original ruler of Hell. Satan lead the second rebellion and took over when he was cast into Hell. Curiously Pluto was banished, and whilst Satan nominally rules from his capitol Pandaemonium, because Hell is so vast there are many Archdemons who rule their own spots essentially (whilst ultimately ruled by Satan).
Morgan Le Fay is King Arthur Pendragon's half-sister, and mother of his only child Mordred. Mordred's son's were all killed and Mordred was slain in battle with Arthur, but Mordred's daughter survived. Her line continues all the way to Hellboy's mother. Hellboy's father is Azazel, a prince of hell.
The Queen of Blood defeats Hecate for the title of Queen of the Witches. Who is the Queen of Blood? Nimue, the Lady of the Lake. She stole Merlin's power but eventually goes mad and worships the OJ. She is killed by the other Witches but is revived in the modern day and attempts to bring about the End.
Merlin and Queen Mab are present, but are minor additions to the story. Baba Yaga exists as an extremely powerful witch, and Kochei the Deathless is one of her servants. She even brutally terrorizes the other Gods in her plane.
Finally, Vril is the Secret Fire of God. The Fire of Creation. Used to create everything that exists, including the OJ and the OH.
So, is anyone seeing any similarities yet? Hellboy was even coming out around the same time.
DRESDEN FILES
My take is this:
The Watchers are another name for the Angels in the Dresden Files. Mac is a former Angel, but not necessarily an Archangel. Anum is the Demiurge, like in the Gnostic faith. In Dresden, this would be Satan/Lucifer. Hades is actually (like in DnD) on Hell's plane, but doesn't serve Hell. He runs his own little company, but is left alone to run it. He just is in the Underworld (which is one big plane), with Hell and Gehenna etc. Satan is the ruler of the plane, but there are many other rulers who rule their own pockets. The difference is both his power and his backing, he has millions of demons at his command and is the strongest being on the plane. However he doesn't actually run everyone, as part of his stick is everyone should do as they want (except when it interferes with his plans). Satan didn't create the Old Ones, like Anum created the OJ, but he does think that humanity shouldn't have Free Will as they endanger all of Creation. So he and TWG have a game, where they test every soul at every Choice. They have rules that mean no direct interference, but occasionally the bad guys cheat. If Satan wins enough souls, humanity will be destroyed and Free Will gone. You might say he is fulfilling his purpose, his original purpose, which might have been to protect Reality. The other Angels that are not Fallen exist in Heaven to protect Reality but also combat Hell, however that's such a full-time war that new protectors of the Gates against the Outsiders were needed. This fell to the first Gods, who had been busy protecting humanity from the first monsters and demons.
Hyperboreans are the first race of man, same as Hellboy. They also became the first gods. Thoth is the Egyptian God of Knowledge, Writing and Magic. He is often linked to Hermes/Mercury, who is the Greco-Roman God of Travellers, Medicine, Thieves, Knowledge, Messengers. This God has also often thought to be linked to Odin, the Norse God of War, Magic, Journeymen, Victory, Prophecy and Death. I propose that all these beings are the same God, the same being from the Hyperborean age. He kept inventing new mantles as needed, and eventually founded his own race of Gods. He may often have been a defender of the Gates, but he also has a dual role in guiding humanity and protecting them from the Demons and Gods. Not all Demons are Fallen. Some of these beings were birthed by humanity's Free Will. How? Humans are naturally psychic, because their souls are part of the first moment of Creation. In someways, they are all linked to TWG. This is why they have such enormous reality-warping power and yet can barely control it or are even aware of it. Their emotions and thoughts and dreams are tied to the power of Creation, Soulfire, and created through fear the first monsters and demons. But they also helped power the first Gods too, the Hyperboreans who fed off their psychic energy. This is why Hades can have great influence in an earlier age, or Odin is weaker as Odin now. Their power source is humans, but if humanity start to forget them or change their minds, they cannot be as strong. So they either must change or die.
Most have thought Hecate is all the Queens of Faerie but I think it is more likely that she is but one of them. A trio of Hyperborean witches pooled their power. Hecate was merely a witch, before her rite of ascension. This is confirmed by Bob in Welcome to the Jungle. This also links with the Hyperborean theory, as the prehistoric runes on Demonreach are linked to the runes on the temple in WttJ. Mother Winter is Baba Yaga, this is clearly established. Mother Summer is Baba Vasilia. But I think Diana, the goddess of the harvest is in Summer. However I think in Ireland she has another name: Danu. The primordial Mother Goddess of the Tuatha de Dannan (literally "the People of the Goddess Danu"). I think Medb/Mab/Morrigan/Hecate is Mab.
Harry is the Beast of Revelation, aka Hellboy.
And Soulfire is Vril.
