Tsotha-lanti is a Kothian wizard, whose reputation for cruelty has given him infamy throughout all of Koth. His mere mentioning is used by parents to scare their children into good behavior, as well as by masters, threatening to sell their own slaves to him. He has no respect for human life, flaying live victims to use their skin as parchment. His home is the Scarlet Citadel, where he performs cruel experiments. In the dungeons below the citadel, Tsotha-lanti has many dark creatures, such as his enormous snake Satha.

Tsotha-lanti, together with Strabonus, king of Koth, and Amalrus, king of Ophir, managed to trick Conan into entering Koth to aid Strabonus against the invading forces of Ophir. When Conan arrived, he found that Amalrus was indeed not invading Koth, but they had fooled him into comming there as part of Strabonus' plan. Being outnumbered, the Aquilonian knights Conan brought didn't flee, but attacked the hosts head on, killing many of their soldiers, but in the end were crushed by sheer numbers. The soldiers are unable to capture Conan himself, who killed anybody that came within his sword's reach. At last, Tsotha-lanti decided to intervene and was able to sting Conan with a small spike laminated with the juice of the purple lotus, paralyzing him.

They brought Conan into the Scarlet Citadel, where they offered him freedom if he signed over his kingdom to Prince Arpello of Pellia. Conan refused, and was thrown into the Halls of Horror deep below the citadel itself.

The forces of Ophir and Koth then went on to siege the city of Shamar, but Conan managed to escape from the Halls and return to Tamar through the help of Pelias. Conan gathered an army and went to meet the invaders. Surprise by the return of Conan, and by the sudden appearance of another enemy army, the invaders were slaughtered. Tsotha fled from the battle, but was pursued by Conan. Then, a eagle came down from the sky towards Tsotha. It scared his steed, so that it threw it's rider off, while Tsotha and Conan were left to battle. Conan managed to decapitate Tsotha, but as the head fell to the ground, the body did not stop moving. Tsotha was still alive, his body searching for his head. Then came the eagle down from the sky again, picking up the severed head of Tsotha and flying away, laughing with the voice of Pelias. Tsotha's headless body ran after the eagle and disappeared behind a grove.