The Antiproton Fusion Catalyst is a weapon of mass destruction used by the Andromeda after she traveled back in time to the Battle of Witchhead. It was developed by Harper who felt that if he could destroy the entire Nietzschean fleet, that Earth would not be enslaved and the High Guard and Systems Commonwealth would survive to combat the Magog that would also ravage Earth.
The Catalyst works by injecting a huge stream of Antiprotons to a certain area. Antiprotons are a form of antimatter, therefore, when they come in contact with their matter counterparts (protons), they annihilate each other. This releases an extremely energetic burst of electromagnetic radiation, typically in the form of high energy gamma radiation and x-rays.
This annihilation reaction, not to be confused with nuclear fusion, consumes 100% of the mass of the reactants (both the anti protons and protons) to produce energy.
This burst of energy from the annihilation is powerful enough to serve as an energy source to cause (catalyze) a secondary fusion reaction if there is matter present.
Andromeda realized that there was a large historical discrepancy in the amount of Nietzschean ships that arrived at the Witchhead Nebula to ambush the remains of the High Guard fleet. and that the crew had to destroy about 1000 ships to correct history. Although Harper hid the construction of the catalyst from everyone, when Dylan found out he initially denied Harper the permission to use his weapon, but utimatively understood he had no choice. Using the device set history on its course as the blast obliterated 2/3 (66.667%) of the Nietzschean fleet, leaving the rest to face the Commonwealth fleet in battle.