Arc 4 of the Ultimorian Storyline centers around a mysterious entity that was forged and created by accident; due to various efforts from pre-existing characters to mess around with the timeline, Eidolon was born as a formless parasite who had the ability to possess characters who have been considered as "misplaced" by space and time for their entry into the Ultimorian Universe for an extended period of time. As a direct result of Eidolon's involvement, the no longer relevant information on character backstories are being erased and replaced with information that is up to date. As this is a temporary means of dealing with Eidolon, however, this has allowed Eidolon to ensnare Chronicler into his control, and potentially cause even further troubles from either within Ultimoria, or even beyond...
While Eidolon lacks any true form of sorts, Chronicler the 7th's design specifically for Arc 4 is considered the closest he has to a true form, considering his most common host is Chronicler himself. Not that this only applies to Chronicler the 7th in this regard, as the Chronicler that is considered an Ultimorian Deity is no longer considered to be the same incarnation of the character. As a result, the version of the character introduced (the one using Mabinogi for images), is considered a separate incarnation of Chronicler. In a sense, Eidolon itself could also be considered a separate incarnation of Omnicron.
While an exact personality has yet to be determined, Eidolon seems to model his dialogue, voice, and mannerisms after Dio Brando of Jojo's Bizarre Adventure. As another possible reference to him, Eidolon's leitmotif is that of Zyuoh The World's from Doubutsu Sentai Zyuohger, as Dio's iconic stand is named The World (世界ザ・ワールド Za Wārudo). Although Eidolon behaves in a rather cynical manner with his words, the manner in which he behaves, especially in his debut appearance, is considered to be highly unorthodox for what he looks like.
In addition to this, he seems to have made replicas of three animal companions of Chronicler's to assist him whenever he gets into a fight. Either taking on their form or using their powers in the form of a stance, Eidolon engages in a stance activation/change by snapping his fingers on both hands, a quirk that Chronicler does not have. Not only that, but Eidolon often uses a lot of hand-based gestures very often, despite the fact Eidolon is perfectly willing to talk unlike characters who rarely, if ever, do so such as Omnicron or White Rabbit.
When replicating the forms of Chronicler's three major animal companions, he behaves in a similar fashion to these three animals when doing so, but maintains some form of gesture he uses while in said form to signal that it's still him. Eidolon also has a tendency to actively talk in these forms, whereas the three individuals he's copying do not do so (or, in Dragora the Dark's case, not very often). Even then, though, Eidolon also behaves in a different fashion than normal when transforming back into a human form.
In Truth and Ideals, it is revealed that Eidolon's voice isn't actually his own; he can steal the voices of others to utilize for himself, in which he speaks perfectly like the person in question. However, despite his text being rendered with Zalgo formatting, his voice can easily, and frighteningly, sound perfectly normal when compared to the malevolent, black cloaked and white masked individual that is Eidolon himself. He also tends to be associated with an ungodly and blasphemous shriek which he uses as a defensive mechanism against various types of threats as well.
As Chronicler is often his most common host form, his first action with his primary host body was to make it distinct from Chronicler, as a result, the types of clothing associated with Chronicler were altered in significant ways (different clothing beneath a robe which is now pitch black instead of partially white as well). Not only that, but because of his new robe, his face is completely obscured by a blank, white void, no different from the Slender Man. While it is implied he still has hair much like Chronicler's own, he never removes his hood enough for this to be seen. As his body is heavily obscured by black or dark colored clothing, nobody can be certain just who exactly he is anymore; is still using Chronicler as a host? Or did he more or less effectively make his current body his own?
While not directly made obvious, the above mentioned implication of hair is that Eidolon still has an actual human face underneath a white mask, which he adamantly refuses to show anyone, and gets extremely hostile when even bothered to remove the apparent mask he wears. Aside from his physical body being entirely covered, he maintains the normal features one would expect from a human with all that is capable of being noticed even with his heavily obscuring clothing. When in combat, and also depending on his current stance, he is either equipped with a longsword and kite shield ("Dragon Stance"), a dual set of differently shaped longswords ("Greyhound Stance"), or a crossbow with a full set of crossbow bolts ready to make use of for ammo ("Falcon Stance").
In his side-story role in the 2016 Revamp of E.S.W.N., he appears in his humanoid form shown on this page, but oddly enough, does not appear to possess sleeves for arms, or even his own weaponry for that matter; all that remains are his black cloak and blank white "face". However, this is also apparently the closest he has to a true form; numerous entities appear in the storyline known as a Gatekeeper, a humanoid-like entity with no or obscured arms and shrouded in shadow. In the revamped storyline, they are basically the sole remaining creator gods, but mostly focus on abilities relating to teleportation or the flow of time itself; these are the creators of the Temporal Void, and said location served as a prison for Eidolon's original self.
Ever since the Ancient Oblivion incident, Ultimoria had been separated from universes in which Ultima had no authority over, as in, had not been involved in said universe's creation. As a result, this significantly altered the backstories of specific characters; the separation was also on a temporal-level as well. However, because of Ultimoria's rather notorious history for interacting with outside universes, the result of this was a lifeform which came to be as a means of desperately trying to survive being ejected from the Winds of Existence as a result of this problem. As a result, the lifeform had absorbed the bodies of incarnations of characters which Ultimoria had no usage of anymore... including a perfectly intact version of the Ultimorian Deity Chronicler's past self. Due to the perfectly intact nature of the Mabinogi version of Chronicler, Eidolon gained power unseen otherwise, and took on Chronicler's body as his primary host while significantly altering his wardrobe to match himself more. This in turn altered the canon version of Chronicler in regards to his simple wardrobe, which was the first dead giveaway to Chronicler that something fishy was up, and that somebody was using one of his incarnations as a body, and thus setting up the events of Arc 4 of the Neo Ultimorian Canon...
Truth and Ideals
First appearing in Chapter 58: Blood Moone, Ash and N catch Eidolon in the dark hours of the morning by surprise alongside Regulus, in which the ensuing brawl leads to Eidolon copying the attacks of Tempest and then Flare to eliminate its threats, only for Reshiram and Zekrom to intervene. As Eidolon attempts to escape, he sends out Necrozma to deal with the group, but not before Ash's Landorus attempts to pin Eidolon down, only to suffer the wrath of the brainwashed Necrozma's Z-Move and get taken out of the fight. As Eidolon flees the fight, the results end up with Lunala so brutally injured that it's forced to revert back into its Cosmog pre-evolution stage, and when Eidolon takes notice of this, he corners Regulus, Arcturus, and Mewtwo just outside of the sanctuary and confronts them about Cosmog, stating that they have something that belongs to him...
After he had disappeared, he was manipulating the minds of Necrozma, Lunala, and Solgaleo into having them attack each other, with only Solgaleo showcasing a heavy resistance to this in which Eidolon responded by using his own powers through Necrozma to summon a black hole of sorts to dispose of Solgaleo, and Lunala is unable to tell Necrozma apart from Eidolon due to the effects he has on its mind. As the night ends, it is revealed that Eidolon had a hand alongside Ghetsis in corrupting Necrozma's mind, and that others much like Necrozma are victims of the two of them. In addition, when collecting Pokedex entries from her friends, Bianca's Pokedex ends up scanning Eidolon which causes a very glitchy message to appear, before the Pokedex suddenly shuts itself off.