Grey Miasma of H'rull
A proverbially sticky substance, the Grey Miasma of H'rull is used (at least, among wizards) in the same way that denizens of Roundworld might use the word quicksand. For instance, Rincewind observes "When one foot is stuck in the Grey Miasma of H'rull it is much easier to step right in and sink rather than prolong the struggle" in The Colour of Magic, when he tells Twoflower that a band of Watchmen are in fact Barbarian heroes.
The exact properties and origin of this substance are unknown, as is virtually everything else about it.
H'Rull is marked on the The Discworld Mapp due hubwards of Klatchistan, in between Vangelmesh, Khan'Li, and Muntab.