The Mass Accelerator Cannon, or MAC for short, was an immensely powerful warship. Approximately the size of a dreadnaught and costing over 5 trillion credits to produce and arm, it was both the most expensive bombardment weapon fielded during the war and the most expensive.
After the Egeo Massacre, the Empire looked for a way to do exactly what it had done--that is, subject an entire system to Imperial rule without the need for a ground assault--in a smaller package.
As it stood, the only way to achieve such a result was a prolonged planetary bombardment by an entire fleet of ships, something that was not very cost-effective. As such, the Empire created the first blueprint for the MAC only five months after the battle at Egeo.
The MAC was essentially a self-propelled Rail-Nuclear Launcher, or RNL. Using railgun technology, it fired a massive (100 meters in diameter) missile carrying dozens of nuclear warheads at speeds in excess of 800km/h. Unfortunately, the energy consumption of the weapon and its coolant systems was so great that it could not be outfitted with any other weapons and had to be escorted by a large number of other ships.
The weapon was so powerful, a single shot took an hour to charge up and when it did fire, it would throw the crew of the ship to the floor. All MAC crew were required to wear full-face helmets while on duty. However, it could only be fired once in a 24-hour period, otherwise the crew on board would be melted to the walls.
The MAC was used for a single purpose: annihilate enemy colonies in a fraction of a second. As displayed at the Siege of Madrigal, it could fulfill that job incredibly well. Unfortunately for the Empire (and fortunately for its enemies) its cost is so great and its production time so long that the MAC would not see mainstream use as an invasion weapon ever.
In addition, should its defensive fleet be breached, it has literally nothing in the way of point-defense, so its only choice is to run.