The Thanix cannon fires a stream molten metal -- a mix of iron, tungsten, and uranium -- that has been shaped into an armor piercing bolt. The mass accelerator propels this stream toward the target at a significant fraction of lightspeed.
The exact firepower of the weapon isn't specified, but it is stated that the Thanix cannon can provide the firepower of a cruiser-sized mass accelerator if mounted on fighters or frigates. Just two shots from the Thanix cannon mounted on the Normandy SR-2 were sufficient to destroy a Collector cruiser. The Reaper weapon upon which it is based can deliver hundreds of kilotons of energy in a single shot, but it is part of a much larger warship.
In the aftermath of the Battle of the Citadel, all of the Citadel powers scrambled to salvage debris from the destroyed Reaper. The turians recovered significant portions of Sovereign's main gun, and within two years they had developed the prototype Thanix cannon. Calling upon his contacts in Citadel Security and the Turian Hierarchy, Garrus Vakarian was able to obtain plans and components to have a Thanix cannon installed on the Normandy.
Specifications for the Thanix cannon proliferated, making them common upgrades for warships and starfighters among the Citadel races.