Entropy

Entropy, a product of the second law of thermodynamics, was the slow, irreversible process of breakdown and decay whereby a complex, organised system inevitably degraded into more chaotic ones.

Being a complex and energetic closed system, N-Space, the main universe, had a finite lifespan and would have "died" some time prior to the 20th century had the Logopolitans not vented entropy from it into E-Space, another universe, via the CVEs which they created. (TV: Logopolis, PROSE: Timewyrm: Apocalypse)

In 1981, the Tremas Master interfered with the Logopolitan project and inadvertently closed the CVEs. A wave of entropy destroyed a large portion of the universe. On the homeworld of the Logopolitans, machines affected by entropy ceased to function and then crumbled. Parts of the Monitor, the Logopolitan leader, disappeared unevenly immediately prior to his actual disintegration. Whole stars and galaxies and many regions of the universe vanished, such as the Traken Union and Logopolis. The Logopolitans themselves were wiped out. (TV: Logopolis)

The Black Guardian was the embodiment of entropy and chaos, opposed by the White Guardian, who embodied order. (TV: Enlightenment)

The Mandelbrot Set viewed Gabriel and Tanith as creatures that generated entropy and tried to kill them, only to be destroyed by them. (PROSE: Falls the Shadow)