It's called a Conversion Matrix gun, whatever the hell that means
TFWX Conversion Matrix: Extracting it's awesome power directly from the core of the Apocalypse's total conversion reactor cluster, the Conversion Matrix functions as a remote acquirer of matter to feed the reaction. Though it has never been tested on the field, the calculations behind it show that it will consume enormous amounts of matter per second - as much as 0.87 megatons - and will increase abruptly the power output of the main cluster. Special containment and dissipation systems have been built around the power core of the Apocalypse, as anticipation to a possible event where such weapon might be used, but the lack of proper field testing makes it completely impossible to predict the exact results of activating the weapon.(from design doc)
basically it can destroy a Sathanas with a single shot (even with minimal power usage can't be used with full power because it's way too unstable it could overheat and destroy the whole ship (full power was going to be used at last mission of MT)