Mevolent was an incredibly powerful Elemental and was one of the very few to turn evil. He lead a faction during The War against the Sanctuary with the goal of bringing the Faceless Ones back to this reality. Mevolent is mentioned often during the books. Derek has stated that Mevolent is indeed dead, though he appears in an alternate reality where he won the war and enslaved mortals.
Before his rise to power he served under Arthur Dagan's grandfather and somehow came across The Unnamed who would later become his master. Mevolent was a dedicated follower of the Faceless Ones, and attempted to open a portal and allow them entry back to Earth. His army was lead by his three generals. Mevolent engaged the Sanctuary starting the war. Although equally matched at the beginning Mevolent's side began to lose. In a desperate attempt Mevolent ordered an attack against the leaders of the Sanctuary. Nefarian Serpine succeeded in killing Skulduggery Pleasant and many other leaders fell with him. They paraded the bodies around demoralizing the Sanctuary and with the leaders dead. Mevolent's side acquired more followers and took the upper hand until Skulduggery came back from the dead. The battle shifted with Skulduggery's return. He later pursued Quintin Strom around the world around the time of The Corpse Attack in Denmark. In 1861 he was rumoured to be injured and had also gone into hiding. It is unknown who or how he was injured just that he died.
Mevolent attempts to find out Valkyrie's connections with the Resistance, and doesn't believe her when she states that she doesn't know anything about them. He is later seen at his castle when Skulduggery, Valkyrie and Nefarian Serpine steal the Sceptre of Ancients.
When Valkyrie later returns as Darquesse, they engage in battle during the fight with the Resistance. Although he is ultimately defeated when Darquesse hits him with a horse, Mevolent exhibited extreme combat skills, defeating and decapitating Darquesse temporarily, almost killing her entirely. Though after she puts her head back onto her body she manages to knock him out by slamming a horse into him.
Mevolent made his appearance when Skulduggery and Valkyrie enter the alternate dimension whilst looking for an escaped Ravel. He then tries to take Darquesse on with a new weapon, and nearly succeeds with the attempt. However, Darquesse and those not originally from the dimension fled, and he is not mentioned for the rest of the book.
Mevolent was an Elemental. He was one of the few that turned evil. His mastery over the elements was unprecedented and absolutely incredible, and he had equal skill with all of them. He demonstrated his mastery over air by expanding the air in Darquesse's throat, causing it to burst and also the ability of flight.
Due to the fact that his underlings included Nefarian Serpine, Baron Vengeous and the immensely powerful Lord Vile, Mevolent is assumed to be a sorcerer of unimaginable strength. When she sees his magic, Darquesse claims that, apart from Argeddion and herself, he is most likely the most powerful sorcerer in existence. It is likely that if Mevolent was to discover his true name he would surpass both Darquesse and Argeddion, likely being capable of taking multiple Faceless One's head on.
During his battle with Darquesse, Mevolent shows incredible speed - moving at high speed to go to one place and back almost instantly - and strength - strong enough to lift a large slab of concrete, and rip off Darquesse's head - fighting techniques and sword mastery. As an Elemental, Mevolent should possess no healing ability, yet was able to regenerate and continue fighting after having his bones crushed by Darquesse, showing that his decision to die once a day has some tangible benefits. Although he did not possess the power of knowing his True Name, Mevolent managed to fight on-par with, and even briefly take an advantage over, Darquesse for the majority of the battle, decapitating her and causing other serious injury, Mevolent would have in fact won had he decided to destroy Darquesse's head instead of leaving it on the ground.
Mevolent is a rather violent and cruel individual as shown in Kingdom of the Wicked. Though his speech and demeanour may seem calm with no malicious intent, his actions and fighting skills as well as his methods of torture betray his almost normal appearance. He was obviously extremely charismatic and a brilliant leader, being able to persuade so many sorcerers to his cause, including the likes of Nefarian Serpine and Baron Vengeous.
Derek said that "He loved puppies. He thought they tasted really nice." Although, it is entirely possible, and extremely likely, that he just made this up on the spot.
Derek has stated that Mevolent's favourite music is Chopin's "Funeral March" closely followed by "Do You Want To Build A Snowman,"however, this was certainly no more than a joke.
In Mevolent's first appearance, he heavily implies that he has eaten newborns before, but only those that have had it coming, though this was most likely meant in morbid jest.
The extreme power that Mevolent was supposed to have wielded has also led to speculation that he both knows, and has sealed, his True Name. However, this theory is disproven in Kingdom of the Wicked, wherein Darquesse cannot sense any trace of knowledge about Mevolent's True Name within him.