Velther, the Dark Dragon King, is a villain in the Dragon Quest spin-off manga Dragon Quest: The Adventure of Dai. He is an incredibly powerful evil dragon, the last sentient one of his kind, and the rival of the series' primary antagonist, the Great Demon King Vearn. In spite of his minor role, he is frequently mentioned and still influences the plot through his right-hand man, the Death God Kill-Vearn.
Not much is known about Velther, except that he is one of the most ancient and powerful dragons in existence, rivalling the God of Dragons himself, to whom he might be related in some ways; just like Vearn rivals the God of Demons.
Velther is described as a fire-breathing dragon with darkness-related powers, who fights by combining his Dark Ki (Life force stemming from his evil nature) and his flaming breath. He is as powerful as Vearn himself, though Vearn likely becomes stronger when he assumes his mightier forms. His soul is apparently immortal, enabling him to resurrect when his body is destroyed.
Like Vearn, Velther resents the main gods and plans to wipe out every specy living on the surface of the world, in order to claim it for his kind and his servants. However, unlike Vearn who plans to obliterate the surface, Velther covets it for himself. He loathes the gods and their servants and despises Vearn, but he can still genuinely, albeit begrudgingly, praise the Great Demon King should he succeed.
At the beginning of time, the God of Humans, the God of Dragons and the God of Demons agreed to share the world between their respective species, placing the Demons and most of the Dragons in Makai: the subterranean demonic world where might is the only rule. Velther fought against Vearn for millennia for the control of both Makai and the surface, until Vearn came up with a wager: they would enter a competition to see who would take over the surface first, with the winner winning his rival's submission.
They then established their dominion over Makai and started to build up their forces. At some point in the past, Velther sent his most powerful follower, the Death God known as Kill-Vearn, to serve Vearn as a "truce gift". As his name implies, the Death God's real purpose was to kill Vearn with a Black Core, (a magical explosive as destructive as an atomic bomb) should the opportunity arise. However, Vearn immediately figured this out and while Kill-Vearn kept serving his true lord all along, he also became one of Vearn's most trusted and dangerous agents.As millennia went by, the dragon kind, which was once a race of extremely intelligent beings, highly proficient in magic was gradually reduced to a specy of mindless beasts, still feared as the most powerful monsters in the world but a shadow of their former glory.
The reason behind this regression is never explained. This facilitated Velther's rise into power among his peers, until he fought and killed his most hated enemy, the Lightning Dragon Borix, for complete dominion over the dragon kind. This battle is remebered in Makai as the "duel of the true dragon".
Around fifteen years before the start of the story, both Vearn and Velther were finally able to make their move. While Vearn waited for the outcome of the war between the Demon King Hadlar and the great hero Avan, Velther launched his attack, only to get opposed by the envoy of the gods, the Dragon Knight Baran.
In the ensuing conflict, Velther nuked an entire portion of Makai with a Black Core but to no avail, and was ultimately slain after a long and gruelling battle. Immediately afterwards, divine spirits from Tekai (the Land of the Gods) sealed his soul in a dragon-shaped, stone statue, which they trapped within a temple in Makai.
During the course of the story, Velther is mentioned in several flashbacks about Baran's and Kill-Vearn's origins. He appears for the first time in person when he manifests himself in Vearn's palace, during the final battle between Vearn and the protagonists. Since nothing can (seemingly) stop Vearn's plan to obliterate the surface, Velther reluctantly congratulates the God of Makai, who explains the nature of their conflict to the protagonists, while gloating about the fact that everyone that stood in his way is now at his feet. Vearn then mockingly offers to attack Tekai to free Velther after the surface is destroyed, prompting the offended Dark Dragon King to depart.
However, after Vearn's death at the hand of the primary protagonist Dai, Kill-Vearn resurfaces and attempts to detonate the Black Core set by the Great Demon King, in order to kill everyone on the surface and claim it in Velther's name. Fortunately, the protagonists kill him and thwart his attempt, ending Velther's threat once and for all and securing a durable peace on the entire world.