Diagon is believed to have visited Earth many years ago and gave humans advanced technology. He is also believed to have sworn to come back to Earth to bring the "Golden Age". He claimed he would bring alien technology so that Earth will not have war, nor disease, just peace. Vilgax was mistaken for, and pretended to be Diagon in The Flame Keepers' Circle.
Diagon is in fact an extra-dimensional demonic entity who tried to invade this dimension with the help of his Lucubra army during Earth's medieval era. He was banished back to his dimension when Sir George defeated him by cutting out his heart with Ascalon, a powerful weapon given to him by Azmuth nearly one thousand years ago.
In the legend of Saint George and the Dragon, Diagon was shown as a large black dragon. Its underbelly, claws and wings were green, and it had narrow red eyes with spirals in them.
When it manifests in the "real world" at the end of the series, Diagon is revealed to be a massive beast resembling a giant green octopus. Its "face" is a large mass of tentacles, with either a much larger tentacle or a tail sticking out of its back. It has brown armor on most of its body, with two hooked projections framing its red eyes. In this "natural" form, Diagon is extremely large; dwarfing even Ultimate Way Big.
Personality
Diagon is a very arrogant being who is shown to be excessively power hungry.
Due to the fact that he himself is power, Diagon sees himself as a very godlike and invincible being who deserves to rule all with his mind control.
Diagon talks down to all others as if they are merely nuisances or specks (even referring to some as such).
Diagon does have a sense of honor to the strong and a sense of caution in dealing with them. He addresses Sir George as "George the Triumphant," due to his past victory against him. He also seems to respect Gwen Tennyson, or at least her power. He sent Vilgax to deal with Sir George and Ben due to the danger that George presented to him.
In The Flame Keepers' Circle, Diagon was first introduced as a legendary alien god who is believed to have brought the technology to humans in the past, and which in its turn to bring "The Golden Years" for Earth. His appearance was revealed in a carving.
In A Knight to Remember, Diagon took control of Gwen and explains to them that if Vilgax gets his heart, he will be able to take control of the entire universe. After Vilgax defeats the Forever Knights and the Esoterica, Diagon takes control of Winston and tells him that in order to be powerful enough to rule the universe, he must break the seal. However, this was a trick.
Diagon's voice was heard in Enemy of My Frenemy, being called by the name Old One, a being who can bring dead people back to life. Charmcaster gave him six hundred thousand souls (from every living thing in Ledgerdomain except herself) and in return, asked to bring her father back to life. Spellbinder disapproves of this, saying that his daughter became a worse tyrant than Adwaita ever was by doing so. Spellbinder says that even though he loves her, he can't stay in Ledgerdomain with her, knowing that his life was bought by the cost of so many others, and decides to leave again. He returns to the dead and the Diagon makes all the souls return to their bodies, saying that "such is the way of magic".
In The Beginning of the End, Diagon's voice was heard again when Sir George demanded to face him, but Diagon instead summoned his herald, Vilgax.
After killing George
In The Ultimate Enemy: Part 1, Diagon was able to turn every human into an Esoterica except for Ben's Team, Julie, and Sir George. Later, he took control of Gwen again and he tried to use her to break the seal. Eventually, he broke the seal himself, thus allowing him to enter Earth's dimension.
Diagon absorbed by Vilgax
In The Ultimate Enemy: Part 2, after Diagon freed himself, Sir George immediately attacks him. Diagon then ordered his herald Vilgax to destroy Sir George, but he is nowhere to be found, and thus Diagon sends his Esoterica to defeat George. While Gwen, Kevin, and George fight a couple of Esoterica, Ben transforms into Way Big, and Way Big transforms into Ultimate Way Big. Diagon fights Ultimate Way Big and easily defeats him with acid rain, with none of Ultimate Way Big's attacks having an effect on him. He then proceeds to fight Sir George and Chromastone. After he defeats Chromastone and kills Sir George, Vilgax and Psyphon return with their machine. Thinking it was a weapon, Diagon used his laser beams on it; however the machine absorbs powers from other living beings and due to Diagon's essence being only power, he was fully absorbed within the machine. Psyphon fired the machine at Vilgax and he received all of Diagon's power.
At the end of the battle, Ben used Ascalon to defeat Vilgax by absorbing Diagon into it.
Diagon is a nigh-omnipotent extradimensional "demon". His heart alone is said to have the power to reshape the universe itself, and he claims that "his substance is power". His abilities include, but are not limited to:
Enhanced Strength:
Diagon is extremely strong, as he was able to lift Ultimate Way Big with his tentacles even after shifting into a relatively smaller form.
Enhanced Durability:
Diagon is extremely durable, taking multiple hits from Ascalon and Ultimate Way Big's attacks without any significant damage.
Flight:
Diagon is able to float freely.
Mind Control:
Like the Lucubra, Diagon can control minds, but to a much greater degree. He took over all of Earth in a matter of seconds, while still being partially trapped by the seal.
Resurrection:
Acid Rain
Diagon has the ability to revive the dead.
Fire Breath:
Diagon is mentioned to have a powerful fire breath, although he is never shown using it.
Power Bestowal:
Diagon can give a portion of his power to other beings.
Laser Beams::
Diagon can fire powerful laser beams from his eyes.
Weather Control:
Diagon can summon acid rain and lightning.
Shapeshifting:
Diagon can freely reshape his own body into a variety of forms.
When battling Ultimate Way Big, he adopts a more serpentine form, with smaller tentacles on his head and a body formed from three large tentacles on his underside.
He is said to have adopted the form of a dragon during his first battle with Sir George.
Diagon can also change his size at will.
Regeneration:
Laser Beams
Diagon can rapidly heal from seemingly any injury. Ascalon's energy waves left slashes on his face that quickly sealed, and tentacles severed by Ultimate Way Big's cosmic beams regrew themselves in seconds.
Diagon can see very small things (compared to him).
Weaknesses
Diagon's essence is power, which makes him vulnerable to having his power (and thus himself) absorbed by another.
Diagon's mind control can be blocked by protecting the head with certain metals, as well as mana. Ascalon can also protect its wielder from mind control.
Despite being resistant to damage from conventional weapons, Diagon is very vulnerable to Ascalon.
Diagon is bound to the rules of magic, as shown when he returned a payment from a refused bargain.
Diagon is named after the mythological Dagon, a supernatural being who has taken many identities such as the Semitic grain and fish god and a major member of the Philistine Pantheon.
Also to note of Davy Jones, particularly his head, also resembles an octopus.
Diagon shares many similarities with the Evil Entity in Scooby Doo: Mystery Incorporated.
Both have been imprisoned for millennia.
Both have corrupted others to free them.
Both have absorbed or been absorbed by their servants however, when Diagon was absorbed, he was absorbed to the point that Vilgax took over his body. The Evil Entity however was only taken over for a moment before he re-gained his body but had a different form.
One major difference is that the Evil Entity was part of the Anunnaki who were sincerely helping the humans, whereas Diagon merely pretended to help them.
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