Key to grate in sewers

Key to grate in sewers is a quest item in.

It's only found during the main quest Pyres of Novigrad after visiting Triss' former home in Novigrad. To find it, enter the sewer entrance just below the Glory Gate archway into the city (the farthest eastern archway under the bridge) and it'll be on the body near some drowners. It's used to unlock the nearby grate to take the back entrance into Putrid Grove.

Associated quest

 * Pyres of Novigrad