I’m sure most of you will have seen this story before in the Books of Sorrow, but today I’m here to cover the events of this battle and, more importantly, the significance of what happened during it. So keep reading, because you might learn something new, as is always my goal. First, some of you are probably wondering “what the heck is the Osmium Throne Incursion?” It is when the Vex entered Oryx’s throne world (called the Osmium Throne) through a Tear made by Crota. This all started when Crota’s sisters were experimenting with death and Oversouls. Oryx, being the loving homocidal father he is, thought it was adorable and told Crota to learn from his sisters and of course the big bloke went and opened a Tear into a reality controlled by the Vex (Savathûn supposedly found a way to control where Crota’s Tear opened up, which means she may have been the reason for all that followed). So the Vex came in and started trying to build while the Hive watched, but they were confused because they did not understand the physics of the Sword Realm or the Hive. So the Vex built a mind called Quria, Blade Transform who figured out what the sword logic was just from observing the Hive for a short while. This deductive reasoning of paracausal power demonstrates the phenomenal intelligence of the Vex, because honestly nobody else probably would’ve been able to figure what the heck was going on in the Hive world. So then Quria has all the Vex start killing Hive to become more power, and they were smart enough to teleport around and avoid the Ascendent Hive that could stop them. They kidnapped Hive and Worms and experimented on them, and Quria eventually determined that the most adaptive method to handle their newfound knowledge of the Darkness was to start worshiping it, so the Sol Divisive subtype was created. It is not known why Quria never tried putting a Worm in Mind Fluid, but using their captives they managed to create the Black Garden’s Heart and put it in the Black Garden (its possible that the Heart is pure Darkness extracted from a Worm and grown to strength). From their time there, the Vex also learned how to make their own throne world, which resulted in the creation of the Vault of Glass, Atheon, and possibly also their ability to set places like the Black Garden separate from our time and space. These creations resulted in the Gorgons’, Atheon’s, the Templar’s, and the Oracles’ ability to manipulate time, as well as the creation of the Sol Progeny to conquer the universe for the Vex. We can thank the Hive for giving the Vex the key to paracausality and for teaching them that the best way to conquer the universe is to kill everything else. Meanwhile the fight is still going on and Quria has anchored itself to the Tear so that it can’t be closed. The Vex inside have grown powerful enough to only be vulnerable to the Ascendent Hive, but any Ascendent Hive that tries to get on the outside of the portal to close it grows too weak (because they are only at full power inside the Sword Realm) and is killed. Oryx has been away all this time so the Worm god Eir gets his attention and tells him to straighten up his home. Oryx arrives back in his realm and starts Taking Vex and drives them out of his Throne with ease, but before closing the Tear he tosses Crota through to prove himself worthy of living after his mistake. Having dealt his punishment, Oryx ponders the Vex and their study of geometry (aka perfect shapes to help them conquer all of time and space) and decides he should do the same, then he turns his attention to his world. Savathûn (who claims to have somehow influenced Crota letting the Vex in) speaks with Xivu Arath and tells her that Oryx’s throne is exposed, but before they can try killing their brother Oryx builds the Dreadnaught by building with the bones of Akka, the hammer of Xivu Arath, and the scalpel of Savathûn and then turning his throne world inside out into it. This day became a Hive holiday known as Eversion Day, which is celebrated by turning things inside out. The Dreadnaught was mobile and could only be entered by direct boarding or through the stationary portals of Oryx’s Court, so his sisters and the Vex could no longer invade. We can thank the Vex for inspiring Oryx to make a space ship capable of destroying any fleet it faces. [i]Later on:[/i] Oryx faced Quria again after his first Dreadnaught weapon test on the Harmonious Flotilla Invincible. Quria laid a trap on the Niche Thought-Ship that the fleet had been guarding, but was Taken when it faced Oryx and was unable to simulate a way to defeat him. Quria was later gifted to Savathûn, probably as another of his cruel jokes. We now know that Savathûn has found a way to use Quria to control some of the Taken now that Oryx is gone. [i]Conclusion:[/i] If the Vex and Hive had never had that first encounter, they wouldn’t have become nearly as powerful as they were when we faced them in the game. Oryx gained inspiration to rule the universe so he would be unchallenged in finding the Final Shape, the Vex became more kill happy and learned how to become strong in Hive worlds, the Vex learned how to develop paracausal powers such as time manipulation, and the Vex created the Vault. If you have any questions on the Incursion, the Vex, the Hive, or anything else related to this post, I will happily answer. Same goes for corrections or comments. For more lore, check out my Archive: https://www.bungie.net/en/Forum/Post/217384398/0/0
I disagree with you that the Vault and Black Garden are "Ascendant" realms. The Vex always were able to hop through time and space and thus different realities. In order to gain access to Oryx's throne, we had to become Ascendant ourselves. All we had to do to get to the Vault and the Black Garden was look in the right places and find what was essentially a key. We never actually manifested a change in ourselves like we did with Oryx. It is impossible to say who came around first (Vex or Hive), but when Crota cut the tear that let them in, both were already well advanced into their histories enough to fight to a stalemate. So the Vex were already well on their way to trying to conquer the universe, so the Vault likely already existed before Crota's cut.