This key is used to open doors in the Necromant House that leads to a broken Magic Forcefield. It teleports you to the room with the famous Void.
You have to "use" the tile marked on the picture in order to get the key.
The Wooden version of this key was created to open a door at the Buried Cathedral, but it was not supposed to have a key number already in use. The new door and obtainable key were changed into Key 3200 on Updates/12.02, but existing Wooden Keys 3701 remained in the game, making them unobtainable from this update on.