Entropy wave

After the Master killed all the Logopolitans, an entropy wave travelled across the universe. Since the mathematicians were no longer around to divert entropic energy to the various Charged Vacuum Emboitments, which would have otherwise prevented an entropy wave from forming, an entropy wave naturally resulted, causing entropy to increase to the point of causing heat death at whatever points the wave touched. Many star systems — including that which was Nyssa's home — were destroyed by the wave.

Background
Entropy constantly grew throughout the universe as time went on. Long ago, the universe passed the upper limit for the amount of entropy allowed within it without all structure falling apart. However, the Logopolitans of Logopolis found a way to use their great skill in mathematics to solve the problem. After building a replica of the Pharos Project, they used it to send out their block transfer computations to create and funnel excess entropy through CVEs, the voids leading from N-Space into other universes, such as E-Space. However, when The Master arrived on Logopolis, he used a sonic projector to cancel out the sound of the verbally calculated mathematics done by the Logopolitans, which in turn stops the entropy from being able to be sent into the CVEs while also releasing entropy back into N-Space, causing a build up in the universe and instability in Logopolis which killed all the Logopolitans, preventing them from being able to resume their calculations once the silence had been lifted, and therefore letting the entropy spread, destroying Logopolis. This spread of entropy throughout the Universe was called an Entropy Wave. (TV: Logopolis )

Effects
The entropy wave travelled across the universe, destroying whatever was in its path, including planets like Traken. The wave ceased to exist when The Doctor and The Master fed the Logopolitans' program through the computer at the original Pharos Project, sending the information back into space and reopening the CVEs like it did on the replica on Logopolis. (TV: Logopolis )