The Poison Waste is a large, marshy area of Tirannwn, south of Isafdar. It is located on the western coast of Gielinor. Needless to say, the name adequately describes the deadly, toxic nature of the sludge permeating this area.
The fastest way to physically go inside the Poison Waste is by using a fairy ring (code DLR), which will teleport players to a small island in the swamp (indicated by the travel icon on the map). This island is used in the quest Fairy Tale III - Orks Rift to unlock the queen's hideout and for unlocking the music track Exposed.
The area's only significant uses to date are the collection of sulphur on its mucky shores, the location where coal-tar can be collected from the sludge itself and the Divination training site to the south. The sulphur is used during the Regicide quest, and the coal-tar is used in both Regicide and Mourning's End Part I. The area features in Plague's End, where the player must cross the stepping stones (added with the quest) to find the Elf Hermit, who is really Lady Hefin, one of the 8 elf clan leaders who are needed to regrow the elf city, Prifddinas.
The fact that the sludge consists of coal-tar suggests that coal is used in the process of modifying Anima Mundi, as is occurring in Arposandra. Then, somewhere in the process, the coal is converted into the waste product of coal-tar and is pumped to the surface.