Although it is debatable if the World Devastators were deadlier than the Death Star, they were undoubtedly more efficient. The Death Star could merely destroy worlds, while the World Devastators could use a targeted world's material resources for the Empire's benefit. A Devastator's primary tactic was to land on the surface of a planet. There, its mighty tractor beam projectors would literally tear the planet beneath it apart, thus making them "planet killers." Thus, the World Devastators functioned similar to the Star Forge, which also harvested materials from a nearby astronomical object.
The deconstruction of the planet and its resources would then be used for both fuel for the Devastator and for its massive foundries which churned out starships, starfighters, and droids which would be sent into battle straight from the assembly line. The internal factories could create anything from a starfighter or speeder to a medium-sized cruiser, given enough resources.
Each World Devastator was controlled by a central droid brain—the sentient crewmembers were onboard to oversee production and handle navigation or weapons systems. The droid brain was responsible for creating the new weapons of war, and was programmed with a massive store of ship types and parameters, able to create a wide variety of vessels for any combat situation.
In addition, the factories could also focus on upgrading the World Devastator itself; a portion of the raw materials created in the molecular furnace was set aside for advancements to the main body. With these unique customizations, no two "mature" World Devastators were truly alike. Given sufficient time and resources, the World Devastators could even manufacture more World Devastators.
These machines' shields were so strong that even concentrated turbolaser fire could not penetrate them, and most times any conflict with another enemy ship would result in the consumption of that ship by the World Devastator.
Three smaller Devastators carried their shield generators on their outer hull, which were vulnerable to starfighter attack. If two of their four "legs" were destroyed, the Devastator would fall to the surface without their power.
The Devastator design went through development in The Maw Installation research center prior to the first Death Star's completion, along with other superweapons, like the Sun Crusher. However, they were developed in secret Imperial shipyards, with Crix Madine believing that they never made it out of the planning stages.
The earliest operational World Devastators were deployed in the Borderland Regions around 10 ABY, where rumors escaped to the New Republic of terrible "city-smashers" destroying worlds. Shortly thereafter, fleets of World Devastators appeared around dozens of Republic-affiliated worlds.
They saw heavy opposition in the Battle of Mon Calamari, where they were used against the ocean world of Dac, the temporary capital of the New Republic after the remnants of the Empire recaptured Coruscant.
The ocean world of Mon Calamari had once been slated for destruction by the first Death Star. With the destruction of that battle station—and its successor—it seemed as if Mon Calamari was free of the Empire's grip. But when the reborn Palpatine launched his attack, Mon Calamari became the first target of his World Devastators.
Led by the massive Silencer-7, the Devastator squad descended on Mon Calamari, smashing the floating cities and shipyards, feeding their factories in anticipation of a New Republic counterstrike. These foundries would produce various assault vehicles including TIE/D fighters and wavewalkers. Admiral Ackbar sent a New Republic fleet to deal with the devastating attack on his homeworld, including the captured Imperial-class Star Destroyer Emancipator and a squadron of X-wings. Yet nothing the New Republic threw at the Devastators could stop their inexorable march.
The World Devastators launched automated droid TIE fighters against the Republic starfighters, and one Devastator headed into orbit to face Emancipator head-on. The Devastator devoured the Star Destroyer in its boiling furnace, forcing the crew to evacuate or become the metal monster's next meal. Luckily, the Rebels had another force in reserve, and sent V-wing and E-wing squadrons to the surface to engage the Devastators. Rogue Squadron attacked and disabled three smaller Devastators. The squadron's actions saved nearby Mon Calamari cities.
During the battle, the assault weapons stopped dead in the water. While the Alliance believed the Imperial commander was too inept to even claim victory with the Devastators, this sudden turn of events was entirely to plan. Luke Skywalker had infiltrated the upper echelons of Imperial command by supposedly allying himself with Palpatine.
From his position as Supreme Commander of the Imperial Forces, Skywalker was able to shut down the Devastators. Unfortunately, their internal factories continued churning out wavewalkers and TIE/D fighters, forcing the New Republic to implement a new plan while they fought the Imperial vehicles.
Luke Skywalker smuggled a wealth of Imperial data into Rebel hands, via the astromech droid R2-D2. The little droid was able to decipher the data, and create a new code that would interfere with the Devastators' droid brains. The virus worked wonders; the Devastators turned on each other, leaving nothing but dead hulks in the water world's oceans. The New Republic captured at least one World Devastator, as one was among the fleet surrounding Da Soocha V, eating up derelict spacecraft. Any craft the Republic had was later scrapped after a public outcry.
The destruction of the World Devastator fleet seemed to bother Emperor Palpatine very little, and unsurprisingly, for he soon unleashed a more terrifying weapon: the Galaxy Gun.