Dagon

Diagon is believed to have visited Earth many years ago and gave humans advanced technology. He is believed to have sworn to come back to Earth and bring the "Golden Age". He said he will bring alien tech so that Earth has no war, no disease, just peace according to the legend. Vilgax was mistaken for and later pretended to be Diagon in The Flame Keepers' Circle.

Diagon is in fact an extra-dimensional monster who tried to invade this dimension with help from many Lucubra during Earth's Medieval era. He was banished back to his dimension when Sir George defeated him by cutting out his heart with the Ascalon, a powerful weapon given to him by Azmuth about a millenia ago.

Appearance
Diagon is shown to look similar to Vilgax's true form; a giant green squid as Ben finds Vilgax in the secret room inside head-quarters of the Flame Keeper's Circle to whom the members of the cult worship as the knowledge bringer. But later in A Knight to Remember, Diagon took control of Gwen and uses her to communicate with Ben that Vilgax will attempt to steal the source of his strength, his heart. After that, Driscoll tells Ben that when the First Knight fought Diagon, he reveals that Diagon's true form is somewhat like a Dragon. Diagon scares Gwen as seen in A Knight to Remember and horrifed Ben, Gwen and Kevin into demanding Azmuth to get them away in Solitary Alignment.

Azmuth and Sir George do not seem to be scared of Diagon. They have probably gotten used to the sight of him.

History
In The Flame Keeper's Circle, Diagon was first introduced and his appearance was revealed in a carving.

In A Knight to Remember, Diagon took control both of Gwen and Winston.

In Solitary Alignment, his shadow was seen in a flashback.

Diagon's voice was heard in The 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 her attitude saying that his daughter became a worse tyrant than Adwaita ever was. Spellbinder says that he can't stay in Ledgerdomain knowing that his life was bought by the cost of so many others and decides to leave again. Charmcaster says no, but Spellbinder returns to the dead and the Diagon makes all 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 demand to face him but instead summoned Vilgax, Diagon's Herald.

In The Ultimate Enemy: Part 1, The Seal was broken, allowing Diagon to enter our dimension.

In The Ultimate Enemy: Part 2, Diagon fights Ultimate Way Big and defeats him with acid rain, but later Diagon is absorbed by Vilgax. At the end of the episode, Ben used Ascalon to defeat Vilgax and in consequence killed Diagon.

Powers and Abilities
Diagon has the power of mind control like the Lucubras, but is much more powerful.

Diagon's heart is said to have enough power to rule the entire universe.

Diagon is shown to have powerful fire breath.

Diagon can revive the dead.

Diagon can fire powerful laser beams from his eyes.

Diagon can control the weather (as seen in The Ultimate Enemy: Part 2), he used acid rain to defeat Ultimate Way Big and he used lightning to kill Sir George.

Diagon can change his size as shown in The Ultimate Enemy: Part 2.

Known Mind Controlled Victims

 * Gwen Tennyson
 * Winston

Appearances

 * The Flame Keepers' Circle (carving)
 * A Knight to Remember (flashback)
 * Solitary Alignment (shadow)
 * The Enemy of My Frenemy (voice)
 * The Beginning of the End (voice)
 * The Ultimate Enemy: Part 1 (First appearance in person)
 * The Ultimate Enemy: Part 2 (Later absorbed by Vilgax and Death)

Trivia

 * 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.
 * Diagon himself appears to be based on the Dagon from H.P. Lovecraft's short stories which subsequently lead to the popular image of Dagon.
 * Diagon shares some similarites with Cthulhu: both are aliens, both have mind controling powers and both look like a octopus. In The Enemy of My Frenemy, Charmcaster calls him "the Old One." Cthulhu's species were known as "The Old Ones."
 * Diagon having no heart is similar to Davy Jones (Pirates of the Caribbean) having no heart either.
 * Also to note of Davy Jones, particularily his head, also slightly resembles an octopus.