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What's the story?
"Udoroth" is a song by A Sound of Thunder about the titular mortal so vile that when eventually killed wound up in Hell as punishment, unfortunately for all those in Hell this mortal slaughtered his way until he became their ruler. It also expressed eventually Udoroth wanted to expand his rule to the Earth as well which would result in either a merger of dimensions or worse simply turning it slowly into a new hellish kingdom bowing to him.
Who is he and what does he do?
Udoroth was once a mortal who conquered villages shown with him burning and killing the men before taking the women as his slaves, after taking over on village one village he had corpses surrounding his throne but as he sat one slave stabbed him in the throat. For his crimes Udoroth was sentenced to Hell to suffer for his crimes but upon arrival he started butchering Demons until he took over the entire dimension. When Udoroth took over he had his fun torturing Demons and damned alike to the point all grew to fear him, eventually he started to eat Demons and harnessed enough power to become one. Enjoying his increased power he simply continued and even abused his new slaves/harem with a whip to make them "sing", one day Udoroth heard a prophecy saying he would find a way to enter the mortal world again. With this info he summoned an army and prepared for a new world to conquer which he would lead the frontlines yo enjoy the carnage firsthand. The only force that was a threat was the Queen of Hell who apparently wanted to protect humanity from his cruelty for she knew it would be a reign of nonstop agony for all except Udoroth himself. As they fight neither seems to get the upper hand with the video ending right as they come towards the final battle for humanity leaving the winner ambiguous.
Easily sets it and surpasses the general baseline with torture, taking women as sex slaves shown by women in chains, destroying entire villages by himself including people on fire and portrayed as so cruel Demons feared him especially after he took over Hell. No one really shows any competition and others don't really get any characterization in the song. Yes these acts are on screen.
Does he have personality/Characterization?
Yes, the video shows him in life being a overall stoic man that gets angry over little things or annoyance but is shown and the song throughout stresses he's a sadist including the line "Flesh and gore are such a bore without the wails". He is also shown power hungry and an egomaniac that isn't content with simply ruling Hell but wants to rule it and Earth for eternity presumably by merging them given the line "To rule the living, dead, with no divide". This is further emphasized in his brief scenes in "It Was Metal" where he abused his minions, took pride in all his crimes and keeping the entire dimension under his complete control even killing minions for slight annoyances or simply enjoyment.
Redeeming features / Excuse
Udoroth has none to speak of honestly. In his song he's portrayed as sadistic tyrant who only seeks to harm and rule over everything he can, unlike some songs like "A Little Piece of Heaven" where its done in mostly dark comedy this is not the case and played very seriously. Now he does make brief appearances in "It Was Metal" which in that one he's a bit humorous in a dark way with how he is shown lifting weights (666 reps), using a small Demon a towel after it tries to hand him a real one, brutally kills a Demon to drink and crushed one after trying to read a book about Mercy (apparently annoyed by the sheer concept) but the thing is this doesn't undermine or downplay his cruelty. The scenes give nothing migrating and in story he's treated with fear and spaced out in between to where it's similar to Joker or Bill Cipher where the audience might laugh but ultimately not seen as such in story. The closest to nice things in either video was only using one Demon as a towel vs killing him as well (threw him away instead) and answering one question which the answer was merely the title of "It Was Metal" which seems to take place before the battle with the Queen.
Personally given the limitations of the medium where characterization generally doesn't get too deep beyond the basics I believe he's a keeper. If he appears in the future and something migrating comes up we can happily remove him but to my knowledge nothing else has been confirmed with him at this point.