This is the version of Diagon that appears on Earth-68, the universe owned by Dioga beta. He appears in the John Smith 10 franchise and in Ryder 10. He is an omni-dimensional being, existing in all the dimensions at once. Each one knows of the events the others encountered.
Diagon is a demon from an alternate dimension, trying to escape after being sealed away by Sir George. He takes control of Gwen briefly thanks to the Lucubra's influence, but his primary vessel is Julie. Julie is accidentally thrown into his dimension, where he gains complete control over her. He leads the Flame Keepers' Circle to obtain his heart and break the seal, getting Vilgax in the process. He controls Julie with the Ultimatrix to fight John, while Vilgax tried to break the seal. George and Brago stop Vilgax, but break the seal in the process, freeing Diagon. The two manage to defeat Diagon, and John frees Julie from Diagon, recreating him in the process. He absorbs it into Ascalon once and for all, giving it back to Azmuth.
It's revealed much later that Diagon left a piece of his influence into Dragon DNA, creating the alien Diagoneir. When used by someone unable to keep the Diagon in check, he breaks free and takes control of the user. Because of this, John rarely uses Diagoneir.
Intellectuary uses his powers to pull Diagon's essence out of Puppet John, allowing him to take control and be free. He promises to destroy John for trapping him so long.
Diagon is attempting to get out of his dimension here as well. The Esoterica serve him again, this time led by Baron Highway. Baron becomes his main vassal, serving him to get ultimate power.
He manages to manipulate and control Ryder, his influence staying in Ryder's head, and then in Diagoneir.
In this Dimension, Diagon is an ancient being, revered by the organization known as Doma, which is dedicated to its revival. He becomes an early enemy of Yami, being one of his strongest foes, along with Zorc the Dark One, which it shares a bond with due to being able to merge with it.
