The single most evil character I’ve ever known is Thorvald, the evil Necromancer. Thorvald was evil incarnate. A rather cheerful evil. He was, in fact, quite honest and open about his black magic, but nobody ever believed him. Seriously, he actually announced things like, “Hey, need more skeletons. Be back soon!” And then he’d go and massacre a village.
His history went that he, as a young elf, discovered necromancy as a child, was told no, did horrible things, was exiled, and then went back with a small army of the dead and massacred all his village. Mommy and daddy were never far away from him now!
He got worse.
Eventually, around lvl 10 or so, the party realized he was a horrible monster and kicked him out. Only, they “merely” imprisoned him in a plane of nigh-infinite binding, with no possibility of escape. Thorvald’s player went on with a new character, happily laughing… until the GM came back with a proposal. See, Thorvald knew a lot of magic, Necromancy and otherwise, and that player knew exactly how to get out. He just hadn’t used some of those powers because they were not as much fun.
Long story short, Thorvald comes back to the extremely awesome high-level party. He’s butchered whole universes, and has aquired powers which let him alter the structure of the universe, giving him essentially unlimited powre over everything. And his sole mission is revenge.
The nicest thing he did was to kidnap the younger sibling of one super-sleuth character, turn the kid into a puppy, and hand the puppy off to a childish, but horribly callous and cruel, demon. Thorvald knew the party would contact the demon for information… soon. But not soon enough. The demon appeared for the party with his new puppy - who had been killed and revived thousands of times by demonic magic. Said super-sleuth even petted his own brother (in puppy form), without realizing a thing, or that the demon was so obnoxious. Yes, this was one of Thorvald’s nicer acts of revenge. They got worse. Cities burned, friends and relations died in horrific fashions (or worse, did not die).
The party couldn’t even fight Thorvald, who simplyu used his power to completely quash their abilities with overwhelming force. Thorvald even gave himself special protections which completely eviscerated their entire arsenal against him. Finally, the party used help from the Demon of Time, who really was quite kind and helpful. Drawing upon the entire Quantum Fan of Possibility, the entire combined force of all future events was just strong enough to overwhelm Thorvald. With this dead, Thorvald was dragged kicking and screaming onto the Wheel of Karma, his powers completely destroyed forever.
The super-sleuth, having realized what Thorvald had done to his brother and that he himself had failed to stop it, let his brother’s tormented spirit pass away. The brother was so broken by the umpteen ressurections that he was wholly incapable, his body kept alive solely by demonic magic, his spirit needing to reincarnate merely to escape the eternal nightmare. The sleuth then killed himself following Thorvald’s defeat.
The party as a whole decided that they may have won the match, but Thorvald got it on the points.