Actually, his wife was a Priestess of Mishakal, the Goddess of Healing. She left the priesthood to marry him. When the kingpriest had the arrogance to ask the gods for their power, to remove evil from the world, Soth was given a chance to save the world from the wrath of the gods (The Cataclysm, where they threw a fiery mountain on top of the city,) because he was the champion of the Knighthood. Anyway, Soth left his manor to stop the kingpriest. On the way, some other priestesses, their heart corrupted by Takhisis, and knowing the importance of his quest, sought to stop him. They told him that his newborn son was not his, that his elven wife was being unfaithful with one of his retainers.
Soth, filled with jealousy, turned around and went back to his manor. As he went into the main hall and started accusing his wife, the fiery mountain fell from the sky and split the face of Krynn. The earthquakes brought down half of the manor. A blazing chandelier fell on his wife, and while she burned, she begged him to save the newborn. He refused.
The manor burned and fell around him.
The next night, instead of finding relief in death, Soth arose again, doomed to be an undead knight, the spirits of the corrupted priestesses (now banshees) retelling him the story of his betrayal every night.