Sewer key is a key and a quest item in the Side Effects premium module.
It is obtained by outdrinking Orel during the Down the Hatch quest.
Associated quest
The Witcher 3: Wild Hunt
Sewer key is a quest item in The Witcher 3: Wild Hunt.
It is found at Deireadh prison during The Great Escape and is needed to unlock the nearby grate door in the sewers.