MEDUSA is a ground-based weapon, controlled by an artificially-intelligent computer system. It is stated in Mortal Engines as taking up the whole of the inside of St Paul's Cathedral, where the Guild of Engineers had rebuilt it using an ancient blueprint and a surviving, though degraded, computer "brain". It is described as having a huge, metallic hood shaped like a cobra's hood. It fires a beam of energy (either sourced from outside the real universe, or the city's generators), resembling a "cat-o-nine-tails" or a whip, at targets up to two hundred miles away.
The target coordinates are input via a control panel at the base of the firing mechanism, using codes from an ancient, poorly-understood US-military document recovered from a military complex in the Dead Continent (North America).
MEDUSA was accidentally destroyed by Katherine Valentine who was mortally wounded while attempting to sabotage it. This prevented MEDUSA from firing, but caused it to overload, destroying most of London.
MEDUSA was used by the ancient American Empire during the Sixty Minute War, along with a multitude of other horrifically devastating weapons of mass destruction.
Thousands of years after the War, London secretly made archaeological expeditions to the Dead Continent (the former United States of America) and gathered the plans and pieces of MEDUSA from an ancient, subterranean US military complex and re-assembled it inside St Paul's Cathedral. In order to obtain the ancient, artificially intelligent computer control system, a fabulously complex piece of technology even by the Ancients' standards, Thaddeus Valentine killed Hester's parents and scarred her face for life.
Katherine Valentine spends most of the first part of Mortal Engines trying to figure out what MEDUSA is. Then, when London is being chased by the city Panzerstadt-Bayreuth, and threatened with destruction, the roof of St Paul's Cathedral opens up like a flower. The cold metal hood of MEDUSA emerged from within and, pulsing with strange, otherworldly energies that once brought humanity to the brink of extinction, fired a beam of destructive power at the pursuing traction city, and destroyed the predator in the blink of an eye, reducing it to a smoking pile of molten slag, its people, history and collection of ancient knowledge, wiped out in an instant. Katherine was horrified, but the rest of London was happy that they avoided being destroyed.
London, guided by Magnus Chrome, then headed towards Batmunkh Gompa with plans to destroy the Shield-Wall and make Central Asia their new hunting ground. Magnus Crome planned to use MEDUSA to become the greatest traction city in the world; if a city didn't deliberately drive into London's mouth then they would unleash MEDUSA on it and salvage its remains. His plan would have it so that when all the Traction Cities were gone, they would turn the entire Earth into a mobile planet that consumed other planets for resources.
However, their plans were thwarted when MEDUSA, given incorrect target co-ordinates by the dying Katherine Valentine, overloaded with energy, its raw power spilling out onto London and obliterating most of the city and its people.