A superweapon was a weapon with capacities far out of line with normal military technology, such as destroying entire planets or star systems, and often requiring great resources to create.
Superweapons were created at various points in galactic history, such as the Mass Shadow Generator used during the Mandalorian Wars. World-devastating weapons often had an unknown origin, such as the ancient builders of Centerpoint Station, via its hyperspace tractor beams, or came from an extra-galactic source, like the Yuuzhan Vong, although the latter’s Vongforming technology more often fell into the category of bioweapons. Yo’gand’s Core could fall under the category of superweapon or bioweapon, or both.
Another famous example was the Star Forge, built in pre-Republic times by the ancient Rakata. Arguably the most powerful weapon ever built, the Star Forge constructed an infinite armada of warships, taking energy directly from the star of the Lehon system, Abo. This had many notable advantages, including that the Star Forge could remain hidden while still conquering the galaxy, unlike most superweapons of the Galactic Empire.
One unique superweapon was the Shawken Device, with the potential to destroy the entire universe, but its method of functioning may have rendered it ultimately useless. Also, it is unknown if it was even operational.
During the era of Ulic Qel-Droma, the Sith constructed a superweapon known as the Dark Reaper. The Dark Reaper drew power from the Force itself, and all of its weapons, both primary and defensive, were powered by the dark side of the force. The thought bomb was designed to kill all force sensitives in a certain area around it. It was used in the Seventh Battle of Ruusan against Lord Hoth and the Army of Light, both to turn the tide of the battle and so Darth Bane could reform the Sith under the Rule of Two.
Rokur Gepta used a bioweapon, the electromagnetic torpedo, to wipe out all life on the planet Tund, and prevent life from ever returning.
The Galactic Empire, via the Maw Installation and Bevel Lemelisk, built the Death Star prototype, the DS-1 Orbital Battle Station, the Death Star II, possibly the Death Star III, the Tarkin, and the Sun Crusher, all before the Battle of Endor. When the reborn Emperor Palpatine surfaced, his loyalists at Byss built four more planet-destroying weapons: the Galaxy Gun, the World Devastators, the Eclipse-class dreadnought, and the down-scaled Sovereign-class Super Star Destroyer.
Superweapons were typically designed and manufactured by interstellar governments, as most minor factions could not afford the materials and technology required to build them. However, at certain points in galactic history, superweapons were commissioned by powerful crime lords. Sise Fromm commissioned the Trigon One, and Bevel Lemelisk designed the Darksaber for Durga the Hutt.
It should be noted that large capital ships, such as the Interdictor-class cruisers, the Confederacy of Independent Systems‘ Providence-class carrier/destroyers, and the Empire’s Imperial-class Star Destroyers, were capable of rendering entire planets lifeless, but not completely annihilating them. Unlike the use of superweapons, this usually required a sustained bombardment, a Base Delta Zero, a combined assault from a fleet of ships, the sabotage of planetary shields, or some combination thereof.
In later years, the Galactic Empire was criticized for its overuse of ultimately ineffective superweapons. Han Solo once discussed how the Empire might have reacted to the Yuuzhan Vong War: «What the Empire would have done was build a super-colossal Yuuzhan Vong–killing battle machine. They would have called it the Nova Colossus or the Galaxy Destructor or the Nostril of Palpatine or something equally grandiose. They would have spent billions of credits, employed thousands of contractors and subcontractors, and equipped it with the latest in death-dealing technology. And you know what would have happened? It wouldn’t have worked. They’d forget to bolt down a metal plate over an access hatch leading to the main reactors, or some other mistake, and a hotshot enemy pilot would drop a bomb down there and blow the whole thing up. Now that’s what the Empire would have done.» ―Han Solo, to Vana Dorja