Thadon

The Duke of Mania, his use of mind-altering substances is well-known. He resides in House Mania's wing of Sheogorath's Palace. He's well recognized to be a 'Master of Merriment', and to be fond of over-indulgence.

Thadon spends much of his time in his throne room or painting in the Halcyon Conservatory. Speaking to him during the Understanding Madness quest will give you the quest Addiction.

Later on the game, if you choose to become Duke or Duchess of Dementia, he will appear to you and Sheogorath, and will announce a threat to the Prince of Madness. Then he leaves to join Jyggalag, and aid him in the Greymarch.