Argeddion (also known as Walden D'essai) is the main antagonist of the seventh Skulduggery Pleasant novel, Kingdom of the Wicked. He is a Mage who had discovered his true name, and attempted to use his powers to make everyone in the world magical - his vision of a peaceful world. However, this plan was ultimately opposed by the likes of Valkyrie Cain and Skulduggery Pleasant, especially the latter, who believed that his plan would result in an all-out war.
Argeddion, then known as Walden D'Essai, was a Mage searching for a way to make the world peaceful. During his studies, he discovered his True Name. During this time, he was dating Greta Dapple. Because he discovered his True Name, Tyren Lament, Vernon Plight, Lenka Bazaar and Kalvin Accord retrieved him and placed him under an enforced coma. The group then travelled to the Himalayan Mountains to keep Argeddion out of harms way.
Whilst in a coma, Argeddion transferred a portion his magic to mortals across the world, believing if everyone had magic the world would be perfect. He also manipulated Tryen and the group during his time in a coma. He also gave magic to Kitana Kellaway, Doran Purcell, Sean Mackin and Elsie O'Brien, with the latter leaving the group and lamenting her powers. He later controls Silas Nadir into leaving an Echo on Valkyrie Cain.
After Fletcher teleported the Cube to the Accelerator in the Irish Sanctuary, Argeddion controlled the minds of Tyren and Kalvin, making them generate an impenetrable force field around the Accelerator. Skulduggery and Valkyrie confront him, after which he reveals his plan and, to prevent them from interfering, seals off their alternate personalities. He then teleports away, having learned how to do so from experiencing Fletcher do it.
Argeddion is next seen when Valkyrie and Skulduggery are rounding up Kitana and Doran, who have gone on a killing spree in a school. This leads to a heated argument between him and Skulduggery about him refusing to accept that his plan isn't working, which ultimately ends with Argeddion accusing Valkyrie and Skulduggery of hypocrisy, due to the nature of their alternate selves.
Argeddion is seen again when Skulduggery and Valkyrie lure him into their presence by confronting Greta. He dismisses her to safety, then moves right above the Accelerator as Skulduggery instructs. The Accelerator then shoots a beam of magical energy, that manages to keep Argeddion in the one spot. However, he summons magic and launches his one beam of energy back through the Accelerator, destroying it and injuring a lot of the mages powering it.
Kitana, Doran, and Sean then attacked Argeddion, in which he seemed to be defeated. The trio then attack the other Mages present.
Kitana then kills Greta, when she tries to make them stop attacking him. In his anger and sadness, Argeddion shatters every mental block in the area, including the one he placed around Darquess in Valkyrie's mind. Skulduggery also finds the case with Lord Vile's armour nearby, Argeddion having hidden it in a shed.
Valkyrie and Skulduggery fly back to the Sanctuary, where Kitana, Sean and Doran are attempting to put Argeddion back into the device that held him in a coma, just so they can keep their powers. Valkyrie and Skulduggery attack them as Darquess and Vile, battering them repeatedly, yet they refuse to stay injured. Valkyrie finds Argeddion feeding them power, but severely reducing his own. Darquesse then starts breaking down Argeddion's defences, but before she quite manages it, Argeddion boots the power of the echo shunt inside Valkyrie. Once in the other dimension, he goes to find his other self.
When Darquesse returns from the other dimension, Argeddion reveals that he has taught his alternate version's true name to him. However, his counterpart refuses to cooperate with his plan, due to living in a world where magic is known to the world, and where the sorcerers are evil, and the weak are kept in rural villages, down in the gutter. When the Alternate Walden tries to force Argeddion to send him back, they have a fight, ending in Argeddion accidentally killing Walden before being knocked unconscious by Skulduggery Pleasant moments later. He is last seen having is memory wiped, and having his true name sealed off to him, in order to be 'reborn' as a mortal.
While he was under the impression that he was a teacher an had no inclination of the magical world (due to Deacon Maybury), Darquesse comes to him pretending to be a friend of some of his students. She breaks down his memory walls by shoving a remnant in him. When she was done, she rips it out of his throat, killing him.
- Despite his intense delusions and plan that, according to Skulduggery would result in millions dead, Argeddion is arguably one of the more sympathetic villains in the series. This is due to the fact he never had any intention of harming anyone and just wanted to make the world a better place. Indeed, Skulduggery admitted that he was a better man than most people, including him, though he added that this is why he didn't understand why his plan wouldn't work. Furthermore, most of the damage caused by his plan was done by Kitana, Doran and Sean and, ultimately, the worst thing he does directly is kill the four people who, in his own words, imprisoned him for thirty years when he had done nothing wrong. He is still far surpassed by the likes of Caelan, however.