The Great Desolatrix

The Great Desolatrix was, as the Fourth Doctor described it, a cosmic storm of unparalleled magnitude and ferocity. He used an analogy to describe it to Leela, comparing it to christmas lights and no matter how carefully one stored them, how they "always end[ed] up in an impossible tangle that takes a month and most of your sanity to unpick". The Great Desolatrix had existed for five-hundred years, cutting off civilisations, snarling up time and space, rendering entire patches of the universe inhospitable. (PROSE: Red Planet)

History
After the Abandonment of Mars, eleven World-Ships were sent out to find a new homeland for the Ice Warriors. However, communication between High Marshal Kryss and the World-Ship he was the captain of was severed from the rest of the World-Ships, due to the Great Desolatrix. With his World-Ship damaged from a crash, Kryss fought on the bridge, claiming that it was his new Mars, slaying anybody who tried to take it, for seven days. However, he was then captured by Sycorax, who took him to an asteroid, forcing him to fight in the Cage. Eventually, after the Fourth Doctor and Leela arrived, searching for the Great Desolatrix, they were captured by the Sycorax, and Leela and Kryss were forced to fight in the Cage. The Ice Warrior tried to kill Leela in rage, but Leela calmed him down, and he then asked for her to kill him. She refused, and then, through the speakers surrounding the Cage, several of the Ice Warrior captains of the other World-Ships asked for immediate help due to damage from the storm. After the Ice Warriors destroyed parts of the asteroid, Kryss was rescued. (PROSE: Red Planet)

Behind the scenes
The Great Desolatrix bears, probably coincidentally, a strong similarity to the Flux, the omnipotent threat of the titular series of Doctor Who. Both are functionally very similar, being cosmic storms that rip through the universe, disrupting space and time.