Thadon is a Bosmer and the Duke of Mania, and lives in House of Mania's wing of Sheogorath's Palace. He's well recognized to be a "Master of Merriment," to be fond of over-indulgence, and by his use of mind-altering substances.
If the Hero chooses to become Duke or Duchess of Dementia, he will appear to them and Sheogorath, and announce a threat to the Prince of Madness. He then leaves to join the forces of Jyggalag to aid him in the Greymarch. It is implied in the quests "Addiction" and "The Lady of Paranoia" that Thadon and Syl were together for a short time.
During this quest, Sheogorath asks the Hero to become a member of the court of his two dukes, Lord Thadon and Lady Syl, in order to gain a better understanding of his realm. Speaking to Lord Thadon will start the Addiction quest, which in turn must be completed in order to finish Understanding Madness.
Lord Thadon asks the Hero to retrieve the Chalice of Reversal, a recipient that is able to remove all ill effects that come from Felldew consuming. Giving the Chalice to Thadon will name the Hero a Courtier of House Mania.
The Hero must remove the influence of Order in Sheogorath's Palace by cleansing the pools of Mania and Dementia beneath the Fountain of Madness. This can be done by killing the Priests of Order, taking the shards in their bodies, and using said shards to shut down the obelisks found on the pools. If the Hero became the Duke of Dementia during Ritual of Dementia, Lord Thadon will be found here, having joined the forces of Order. As such, his death is required.