Note: Massive spoilers for The Clone Wars series, Star Wars: Rebels, Ahsoka, Tarkin, A New Dawn, Lords of the Sith, Lost Stars, Catalyst, and Rogue One: A Star Wars Story.It's finally here! After what felt like forever and a day, Rogue One: A Star Wars Story has finally arrived stateside and has more or less garnered universally positive reactions by casual and hardcore fans alike. This is due to the film effectively doing what the Prequel Trilogy tried to do but did not totally accomplish, which is fill in the gaps in the canon. Rogue One was more or less outlined in its entirety in the Star Wars opening crawl, but the new movie gives some amazing context, like what's going on in that scene from Episode IV where Vader chokes out Admiral Motti. Here's what you need to know about the new Star Wars canon!
In arguably the most informative book in the new canon, Catalyst by James Luceno, we learn a key part of the Death Star's construction timeline - exactly when it begins. Shortly after the Battle of Geonosis, the Republic somehow obtains (see: Palpatine's puppetry) the battle station plans Count Dooku/Lord Tyranis took from Poggle the Lesser at the end of Attack of the Clones. Under the impression that the Separatists were constructing their own mobile battle station, Palpatine commissioned a similar battle station for the Republic itself, complete with super weapon capabilities. For the next three years, the Republic entered a nonexistent arms race.
Approximately one year after the events in Attack of the Clones, the Prime Meridian of the Death Star was already completed as construction continued in the asteroid field surrounding Geonosis, documented at great length in Catalyst.
Kyber crystals, for canon newbies, are a type of "living" crystal found within the Star Wars galaxy that are deeply connected with The Force. Typically, they are utilized in lightsabers by both Jedi and Sith, but given their nature and link to The Force, each Kyber has vast potential when it comes to harnessing the energy of The Force itself. This resulted in the development of superweapons, not entirely dissimilar to the Death Star, by the ancient Sith, which made the monopolization and control of Kyber crystals a priority of the Jedi Order.
One of the "mainstream" introductions to these crystals is in a two-part Clone Wars arc, "The Gathering" and "A Test of Strength," where a group of younglings led by Ahsoka Tano travel to the ice world of Ilum in order to seek out a crystal to construct their first lightsabers. Later on, in an unfinished - but still 100% canon, according to Pablo Hildago, canon tzar at Lucasfilm - three-episode arc called "Crystal Crisis on Utapau," Obi-Wan and Anakin discover a boulder-sized Kyber which they try to keep out of the hands of the Separatists, fearing the damage it could cause if harnessed against the Order.
After Order 66, the Empire established mining colonies on worlds rich with Kyber crystals, namely Ilum, Utapau, and even Lothal. Further, the Empire also demolished ancient Jedi sites of worship, not only to suppress symbols of resistance, but to further harvest additional Kyber crystals to power their Death Star project.
One of the biggest flaws the prequel trilogy maintains to this day is the lack of overall scale regarding both the size and political complexity of the galaxy. The Clone Wars animated series helps fill in the map in ways that totally redefine how viewers perceive the saga films as a whole. The greatest gift the series grants us is the perspective of galactic citizens that fall somewhere between the Republic and the Separatists - the everyday person (or weird alien thing).
These are beings that simply want to exist and not be bothered by either side at all, leaving the war to people that don't directly impact their way of life, showing that the line between good and evil, right and wrong, is really quite smudged and often gray.
In Catalyst, we learn about how the early stages of the Death Star's construction took place over the planet Geonosis, the site of the first major battle in the Clone Wars. From a logistics standpoint, this makes sense - the Geonosians had suffered a massive loss and did not have a way to combat the encroaching Republic construction sites. We then find out the Geonosians were used in resource extraction from the surrounding asteroids as well as the battle station's construction in conjunction with droids.
At least three years before the Battle of Yavin, the Death Star's construction site was abandoned for another site. Due to this, the Empire, in order to maintain a massive level of secrecy about the project, eradicated the entire population of the planet, killing approximately 100 billion citizens.