Lord Soth of Dargaard Keep is a Death Knight from the world of Krynn and one of the oldest undead in the setting, as he predates the Cataclysm. Created by the Hickman power couple, he is generally held up as being one of the greatest D&D villains of all time; in Dragon Magazine #357, the last issue, he was ranked in at #15 on the d20 Greatest Villains, making him the 6th best villain the franchise has ever seen. Among those keeping our boy off the top five was the first place winner, fellow Gothic undead villain ... Strahd von Zarovich. Also by the Hickmans. Can't lick the Hick.
As is sadly to be expected for someone from a D&D setting driven by novels, Soth's backstory has been retconned, mostly in the name of amping up the grimdark including to make him more evil before he met his wife ... which, of course, makes him less tragic. We'll stick with the Hickmans' original vision for baseline.
The Tragic Backstory
Lord Soth was once a Knight of the Rose, the most prestigious rank of the Knights of Solamnia, until the day that he encountered ogres trying to murder and/or rape a band of elf priestesses of Paladine in the forest. He slew the beasts and then rescued the elves, but tragedy struck; he and the youngest, most beautiful priestess fell in love and began having sex with each other. The problem was, Soth already had a wife back home. Consumed with lust, he sent a secret message to his seneschal, Caradoc, and had him murder Soth's wife so that Soth could secretly take the priestess as his wife.
Naturally, the disappearance of Soth's wife was deemed suspicious and he was imprisoned, his guilt assured when the elf-maid's pregnancy by Soth was revealed by her three fellow priestesses. Soth was to be executed, but his wife and still-loyal men helped him escape and flee to Dargaard Keep, where they were beseiged by the rest of the Knights. During this time, Soth's temper grew increasingly foul, until one day he struck his hugely pregnant wife; horrified by what he had done, he prayed to Paladine for redemption, and was offered a chance; he was told of the approaching Cataclysm, and told that if he were to undertake the potentially deadly quest to stop the Kingpriest before he gave the gods no choice but to unleash it, his honor would be restored.
Soth told his wife and set off, but fate tragically intervened; the same three elven priestesses who had revealed his secret love affair accosted him on the road. They had found out about his quest - how, nobody knows, because surely his wife wasn't stupid enough to tell these bitches anything.
To explain what happened next, you need to understand something: the Kingpriest's plan wasn't entirely his own idea. He was secretly being manipulated by a sect of elven bigots, who truly believed that their race being created by the Gods of Good at the dawn of creation meant that they were blessed above all else. These elves believed that through the Kingpriest, they could scourge the world of all the evil races - which basically amounted to "everything but elves and their human pawns". So, these crazy bitches ignored the whole "the world will burn in divine wrath" part of the story and instead resolved to stop Soth by whatever means possible.
Doing so turned out to be disappointingly easy. Soth was many things, but he wasn't very bright. They spun him a bullshit story insinuating his wife had been cheating on him and that the baby wasn't his, and Soth immediately turned around and charged back home. He stormed into his wife's chambers, where his wife had given birth to his son not so long ago, and began accusing her of infidelity even as the Cataclysm hit the planet. Fire crashed down on his wife and she begged him to save their son... but Soth refused, still believing she had been unfaithful. With her dying breath, she cursed him and the gods took heed, devastating Dargaard Keep and turning Soth into a Death Knight and his men into Skeleton Warriors (a beefier sort of Skeleton).
At least those elf bitches got theirs; they got turned into banshees and condemned to haunt Soth's lair, constantly singing the story of how they were all damned to this state.
Soth first made his real mark on the world as part of the War of the Lance, when he allied himself with the Dragonlords of Takhisis at the behest of the Blue Lady, Kitiara. In reality, he had nothing but contempt for Takhisis; he followed Kitiara because he had fallen in lust with her and wanted her for himself. As a result, at the end of the War, he waited for her to perish in battle and then took her body before fleeing back to Dargaard Keep.
Whilst he did this, he had his seneschal, Caradoc, who was condemned to eternity as a ghost, visit the Abyss to retrieve Kitiara's spirit. Unfortunately for them both, Caradoc tried to blackmail Soth into giving him payment for Kitiara's spirit; in the resultant rampage, the mists of Ravenloft reached out and swallowed them both, ultimately leading to Soth damning himself to a prison in the new domain of Sithicus. There, he ruled over a populace of tormented elves and kender vampires, but his home was a permanent ill-fit, and the banshees that followed him singing the story of how he was damned kept getting the details wrong until even he couldn't remember what happened. It doesn't sound like much but even in death he was still a man accustomed to rigid military order, and it actually drove him nuts.
Eventually though, Soth realized what few others (and no other Dark Lord) ever had: the only way to win at Ravenloft's Dark Powers' game is not to play it. Rather than rage impotently at a domain that tormented him, he sought solace in oblivion. He had multiple magic mirrors crafted that allowed him to dwell in magically shaped memory-worlds that were either close enough to the truth for his peace of mind, or pure wish-fulfillment. Being undead he could spend his entire time locked in such dream trances, completely ignoring whatever happened in Sithicus and relatively happy. In the end, though, a combination of an uprising instigated by his rivals, the intervention of his late wife's ghost effectively re-enacting the destruction of Dargaard Keep and the will of the Dark Powers (who were actually a bit miffed at him giving them the middle finger with his dream trance escaping his punishment) forced him out of his dream mirrors for good. And he accepted it. He accepted the punishment set before him, and accepted all the evil he had done. So the Dark Powers sent him back to Krynn.
Dealing with Ravenloft's Dark Powers was already one hell of a feat, but once back where he started he did even better: he eventually attained redemption by basically flipping off and triggering Takhisis so hard she proceeded to restore his mortality for the special purpose of having the satisfaction to kill him herself. Bitch was royally pissed off!
The Hickmans infamously were furious about Soth's stint in Ravenloft, as they have long hated the idea of "crossovers" and so despised the setting that had sprung up from their original Ravenloft modules. They have constantly tried to retcon Soth's trip out of canon in their novels, but thanks to the screwy way time works there, the Ravenloft writers were free to ignore it until they purposefully eradicated one of the most iconic villains in D&D through divine intervention so that no one else could ever have him. Despite this, in his Dragon Magazine "hall of fame" appearance, it is pointedly noted that Soth spent time damned to the Demiplane of Dread as the lord of Sithicus. Additionally, it is said that the current Darklord of Sithicus is at times tormented by visions of the redeemed ghost of Lord Soth.