The Mass Shadow Generator was an apocalyptic piece of technology capable of fracturing planets and destroying all life. It was created during the Mandalorian Wars and was a focal point during the finale of Knights of the Old Republic 2: The Sith Lords.

It was created under Darth Revan’s orders by the Zabrak Bao-Dur, companion to Meetra Surik the Jedi Exile. Created on the planet Malachor V during the later stages of the Mandalorian Wars, it was used to absorb the Mandalorian Fleet into the destablising planet in order to bring an end to the war. However in it’s cataclysmic victory as the Mass Shadow Generator activated, engulfed the planet and nearby orbital ships, ravaged the body of Nihilus, turning him into the twisted Darth with an insatiable hunger for feeding on force power.

Years later, Malachor V and the MSG would find itself at the forefront of the Battle of the Traus Academy, where the Jedi Exile and her companions defeated both Darth Sion and Traya, before ordering the Mass Shadow Generator online once more, obliterating the planet into millions of fragments.

Over three-hundred years later during the renewed galactic war between Jedi and Sith, after the Invasion of Manaan, a fanatic Sith Cult lead by Lady Dominique attempted to rebuild a Mass Shadow Generator for their own nefarious purposes, embedded within an asteroid chunk in the space sector known as R-4. This base contained many painted Sith banners, baring a stylized emblem of the Mask of Darth Nihilus.
This new base was not only helmed by warmongering droid machinery, but also Sith Lords that had augmented themselves by fusing force powers and technology, giving birth to a twisted biomass that spread thoughout the R-4 base.

Sources:

The R-4 Anomaly Raid – Star Wars the Old Republic: Legacy of the Sith 7.1
Galactic Timeline Video 8: The Jedi Civil War
Galactic Timeline Video 9: The Mandalorian Wars
Star Wars: Knights of the Old Republic 2 – The Sith Lords
Star Wars: Knights of the Old Republic