Star Wars has set up what seems to be a new Sith superweapon - one that still exists in the time of The Mandalorian. The Sith have always loved superweapons. In Star Wars: The Clone Wars, Master Yoda remembered the darkest days of the Sith Empire. "Long ago in forgotten times," he observed, "when the Sith and Jedi fought for control of the galaxy, weapons there were, of unimaginable power." Many of these Sith superweapons were powered by kyber, although that particular secret seems to have been lost in the mists of time. However, it now seems that the franchise has reintroduced the concept with an all-new design.
The secret to superpowered weaponry was rediscovered by the scientist Galen Erso, and the Emperor wasted no time in creating a wave of superweapons. The first Death Star destroyed Jedha and Alderaan, and a second was almost completed over the forest moon of Endor. Meanwhile, Imperial scientists set about converting the planet Ilum into Starkiller Base, a project that was only completed decades later, in the days of the First Order. Meanwhile, miniaturized Death Star technology was installed on the Emperor's hidden Imperial Star Destroyers in the depths of the Unknown Regions. Palpatine was obsessed with power, and the only way he knew to rule was through fear, meaning he had a wave of destructive superweapons constructed. There have been hints the First Order had other superweapons, concealed in the Unknown Regions but never deployed.
Timothy Zahn's novel Thrawn Ascendancy: Lesser Evil suggests an ancient Sith superweapon still exists; Starflash, a device similar to Starkiller Base that draws on the power of a star in order to project a destructive energy beam. There are hints the Chiss took Starflash from the Sith, who they had confronted millennia ago when the Sith Empire flourished in the Unknown Regions; although Starflash seems to be a single-use weapon, the Chiss acquired two copies. They deployed one Starflash, drawing on the energy of the star their homeworld Csilla orbited; while it was effective, it left Csilla in the grips of an ice age, with the sun's energy output permanently reduced. The other has been kept secret by the Chiss, with only a handful even knowing of its existence.
Grand Admiral Thrawn is one of the few Chiss to know about Starflash. A skilled military tactician, he was assigned to a false exile in order to enter the Empire and assess whether they could serve as allies for the Chiss. Thrawn remained with the Empire longer than he had expected, but was blasted back into the Unknown Regions in the dramatic finale of Star Wars Rebels. He appears to have returned in The Mandalorian era, with former Jedi Ahsoka Tano believing Thrawn is responsible for organizing an Imperial resurgence in the Outer Rim. If so, it's possible Thrawn could yet draw upon Starflash to deal with his foes.
Star Wars has always been associated with superweapons, and it's interesting to see one in the hands of someone other than the Emperor. Grand Admiral Thrawn is a master tactician, and he'd make the most of his one shot with Starflash, potentially doing phenomenal damage to his enemies and threatening The Mandalorian time period. The New Republic should be grateful the Chiss haven't been able to recreate it - likely because they lack the kyber to do so.