Star Wars: Episode III - Revenge of the Sith. The sequel trilogy launched with the introduction of a new superweapon, Starkiller Base, which was essentially the Death Star on steroids.
Starkiller Base was an entire planet that had been converted into a mobile battle station by hollowing out its core, and it was armed with a superweapon that consumed suns as a power source. The the Starkiller Base planet was Ilum, an icy and lifeless world that was unusually rich in Kyber Crystals. And now the Star Wars: The Rise of Skywalker Visual Dictionary has tied everything together quite neatly.
The Star Wars: The Force Awakens. This confirms that the Empire discovered the planet Ilum a year after Palpatine took the Imperial throne. The Empire immediately began Terraforming and strip-mining Ilum, converting it into Starkiller Base. It seems that Palpatine's Empire was constructing Starkiller Base at the exact same time they were building the Death Stars.
On the whole, this makes sense. The The Clone Wars proved that the Jedi hadn't even begun to deplete the vast deposits of Kyber.
And yet, ironically, this does cause major continuity problems. three failed Starkiller Base tests now clash with the Star Wars timeline; when exactly were they conducted? Ilum was the prototype, which means it should have been the first failed test.
