Ketheric Thorm is a half elf who rose from the dead to lay siege to Baldurs Gate. In life, he was a kind family man who was also the mayor of a medium sized town. He was devoted to the goddess of light and the moon, Selune, and built a tower called Moonrise Tower in her name. After the construction though, his wife and daughter both passed, causing him to become bitter and turn to the goddess of loss, Shar. He demanded that everyone in the town turned to worship Shar, or be executed. Even though a few did, there were still a small group who would worship Selune in secret.
Eventually, another group called the Harpers would hear of Ketheric Thorm and his actions. The Harpers were a group that fought against villany and sought alliances to end his reign. Together, with a group of druids called the Emerald Enclave, they defeated the army that Ketheric Thorm had created and killed him. Before his death, Ketheric Thorm cursed the land, causing all not protected by Selune’s blessing to be overtaken by shadows and killed. Eventually, Ketheric Thorm was revived by Myrkul, ex-god of death, and told to raise an army to siege Baldurs Gate in exchange for reviving his daughter. Unfortunately, his daughter ran, once seeing the man her father had become, and Ketheric Thorm focused the rest of his efforts into attacking Baldurs Gate until he is eventually defeated by the player and their party.
Here's a video expaining why Ketheric Thorm is my favorite antagonist of the three.