u have to kill devius in medusa's room, that is after killing medusa..after that the bart leslie, lestie or sumthing will play his song and the mist dissapears..

After you get back from killing Medusa at the Altar of Time you return to Devius's manor
and kill him at the top of the clock town. Afterwards, when you leave the manor you'll
find a bird that is Lester. After Lester returns human, he will play a song that will get
rid of the Miasma.