I’ve seen this question come up a few times on various messageboards I visit, so I’m reposting one of my replies:
Lord Soth is one of the truly iconic Dragonlance characters.
His main story can be found in the Dragonlance Chronicles and the Dragonlance Legends, but here is a summary for those who want a quick answer.
Lord Soth was a Solamnic Knight, one of the truly great knighthoods of any world. He fell in love with an elf maiden – an acolyte of the good gods. They had an affair, a bit of a problem as Soth was married at the time.
He disposed of his wife, and brought the elf maiden to live with him. Horrified by his crimes, he pleaded with Paladine to let him redeem himself. He was told a way to stop the Cataclysm – a disaster that would almost destroy the world.
On his way to stop the Cataclysm, a group of elf women met up with him and told him that his new wife had betrayed him. Abandoning his mission, he returned to confront his bride. She denied that she had been unfaithful.
At that point the Cataclysm occured (a fiery mountain hit Krynn, killing a huge number of people and sending the world into a dark age). With the castle falling about them, the elf-maid cursed Soth just before she died. Soth died, but rose as a Death Knight.
Fast forward three hundred and fifty years.
The Evil Dragons had returned to Krynn. Led by the servants of Takhisis, queen of dragons and the evil gods, the Dragonarmies began conquering the world. A young upcoming warrior in the Dragonarmies, Kitiara uth Matar (half-sister to Raistlin Majere), was brave enough to stay the night in Soth’s castle and win his loyalty.
The Dragonarmies were stopped by the efforts of a group of heroes led by Tanis Half-Elven. Tanis was a rather conflicted young half-elf, in love with both Kitiara and Laurana, an elf-maid. Of course, Kitiara sent Lord Soth to kidnap Laurana, which he was successful in doing. (This was mainly because Laurana was the leader of the armies of good, not because of Tanis, but it was an added bonus). Eventually, Tanis managed to rescue (and wed) Laurana, but Kitiara and Soth escaped with the remnants of the Dragonarmies.
Kitiara now had command of Soth (who had conceived of an unholy love for her), and the remaining Dragonarmies, but was frustrated in her plans for world domination by the Whitestone Armies (those good armies that Laurana controlled for a while).
Then an opportunity arose: Raistlin, her half-brother and one of the companions of Tanis that had helped defeat the Dragonarmies, had turned to evil and was planning to become a god.
A bunch of plots began, mainly involving the defeat of Raistlin (due to the betrayal of his apprentice, Dalamar, who also became Kitiara’s new lover). However, Lord Soth wanted Kitiara dead so she could become his new bride… and plotted against her, making her think that Dalamar wanted Raistlin to succeed.
In the end, Dalamar killed Kitiara, who almost killed Dalamar (stopped due to the intervention of Tanis Half-Elven), Raistlin was stopped, and Lord Soth gained his prize.
Lord Soth is one of the most dangerous beings on Krynn. In his original incarnation, he could turn gates to ice and shatter them, kill people with a word, and commanded a troop of skeletal soldiers.
Lord Soth appeared in the Ravenloft demi-plane for a few novels, before returning to Krynn, the Dragonlance world. His eventual fate is described in the War of Souls trilogy.