The zeugl's lair, or nest, is the scene of the last boss creature battle in The Witcher (PC). It is located in a part of the sewers in the Temple Quarter which was previously inaccessible. The entrance to this area is found by going through the door immediately east of the Dwarven blacksmith shop in the nonhuman district and then descending to the cellar.
The door is locked for the duration of the game. The key to unlock it is obtained only during the Epilogue from a wounded Order soldier lying in the street, or from Triss.