A major instance that took place several years prior to the Events of B/C, it involved an overly insane Rainboramon stealing the DNA of the most powerful creature alive from the B/C universe and fusing it into Yggdrasill's DNA AND fleeing from a Galacticmon at the same time. Due to trying to experiment and flee at the same time, many time paradoxes and accidents occured because of it's presence as it fled between multiple universes in a chase that lasted 72 hours, traveling to nearly every Digimon universe to ever exist and the chase finally ending at the Adventure 02 universe approximately 8 years after Armageddemon's defeat. The entire event as it took place is an unlockable, move length cutscene rendered in B/C's graphics. It explains several plot details, such as the changes that occur in the other universes, and the early days of the Digimon Dragoramon. Some say this event was all fabricated by The Grand One himself by his own selfish ideals, but some say it was an already highly unstable Rainboramon that led to these incidents.
Prior to the Event
The Grand One was only beginning to form the B/C Universe, as it was then, and still is, in an incomplete condition. The Dragoramon line was created as a parallel to the Nagadramon, Arclimon, and XILA-Mon lines, as they all had lines going down only to Champion, ending as Mega, and required trading between versions at least once for an individual to reach final stage. Dragoramon, however, was unique out of those four. Dragoramon was based on a more original concept of a nigh powerful deity that could alter reality itself, change history as it pleased, and was nearly invincible, due to having been created from a long dead Dragon deity from the same universe that had the exact same abilities except far more advanced. Dragoramon's entire line featured these abilities, but the entire line was universally unstable as a result in order to balance it out. At the same time, however, it could lead to disaster if it had escaped to other universes, which early on, few knew that Dragoramon was capable of this so EASILY. Things were quite until Dragoramon's previous two stages had to be designed, and thus Rainboramon was created. Having more of an actual personality compared to Dragoramon and being able to speak actual lines instead of dead silence, it quickly discovered it's own abilities by stealing a DNA sample of an extremely dangerous lifeform and Yggdrasill, as it fled from pursuit by three Galacticmon, Rainboramon traveled between dimensions to escape from their pursuit.
Side effects of the event
Due to Rainboramon's unstable nature, it could cause unnecessary damage to various timelines if it entered a specific universe at a specific time frame. To make matters worse, it was doing experiments on the stolen DNA and Yggdrasill's DNA AND fleeing pursuit at the same exact time. That being said, these were a mere taste of some of the side effects of Rainboramon's presence:
-The erase of the events of Young Hunters as if they had never happened, and that Tagiru never existed. The events of Xros Wars, however, took place normally. Young Hunters is a mere alternate timeline as mentioned by any Rainboramon if an Arresterdramon has recently been added to a DigiFarm. He'll go on to say, however, that he didn't cause the destruction of certain characters and timelines directly. None of the changes except for the ones relating to the Adventure 02 Universe happened to his own plans, so most of the changes seen from B/C can be attributed to accidents or paradoxes due to the entire Rainboramon line's unstable nature.
-Leomon still being alive in the Tamers Universe.
-Leaving a stable dimensional rip between the Data Squad Digital World and B/C's Digital World allowing travel between both easily.
-Gaining almost enough energy to digivolve to Goradramon.
-Causing several B/C exclusive Digimon to inhabit various other universes, as ALL B/C Main storyline Digimon were descended from this Digimon. SuperMechadramon appearing in the Adventure 02 Universe, the False Sovereigns in the Tamers Universe, Theridramon in the Frontier universe, Cobaldramon in the Data Squad Universe, an entire army of Nagadramon and XILA-Mon lead by another SuperMechadramon in the Xros Wars universe, among other things appearing outside of these more major instances.
-A freak accident that nearly destroyed the Digimon World: Dawn and Dusk universe by a mass summoning of 5 Lucemon Shadowlord Mode. This, however, ended humorously since all five of them were too focused on destroying each other than destroying everything else. Not only that, but they were all feral minded at best.
-Minor or severe personality changes in the main characters of certain universes, but this might not have occurred at all despite rumors.
The most lengthy part of the incident that lead to the creation of the Dragoramon seen in B/C's universe, however, took place in the Adventure 02 timeline 8 years after the defeat of Armageddemon.
The Rise of Dragoramon
At this point only one of the three Galacticmon remained as the other two were destroyed by Rainboramon's increasing power. Arriving several miles above Earth's atmosphere, Rainboramon decide the chase has gone on far long enough and knocks the remaining Galacticmon crashing down to Earth, crash landing right in front of Matt and Sora, who are both 24 years old and married in this time frame (Everyone's ages are based on the Dub, but the mere two year or so difference hardly matters), and taking them both by surprise. Matt was surprised that a Digimon as large as Galacticmon went unnoticed for so long, with Gabumon and Biyomon arriving on the scene from them having noticed something was not right. Flying down to them soaring and hypersonic speeds is Rainboramon intending on finishing off Galacticmon, having known Galacticmon could handle far worse than a mere crash from high orbit.
Before Rainboramon has a chance to do anything, however, Gabumon and Biyomon both warp Digivolve to MetalGarurumon and Phoenixmon respectively to attack Rainboramon, who respondes by forcefully degenerating them to Fresh and leaving them unable to digivolve. Rainboramon, somehow recognizing the surroundings as the famous Adventure universe, begins to make his own legend in the making and flees to the U.S. where Cody, Ken, Joe, are at due to Mimi insisting on them coming along with her. However, it is revealed Mimi was somehow possessed by Rainboramon and lured them to exactly where Rainboramon would show up. Palmon suspected something was not right with her from the very beginning, but didn't know how to react sensing that she could easily be possessed by Rainboramon as well, or so Rainboramon blurted out to her mind sensing she was suspecting something.
Seeing her as no longer useful, when he arrives on the scene he forces Mimi to pass out as Palmon, Gomamon, Wormmon, and Armadillomon arrive and warp digivolve to Rosemon, Plesiomon, GranKuwagamon, and Vikemon respectively. Suddenly Rainboramon seems to mimic Mimi's personality oddly from having possessed her mind a bit too long in his case, and while also forcing their Digimon to Fresh stage and making them unable to digivolve, completely changes their Mega stages to entirely different Digimon (Rosemon to Lotosmon, Plesiomon to Vikemon, GranKuwagamon to BlackImperialdramon, and Vikemon to GuardiAngemon). They were briefly in these forms for a moment before he knocked them back down to Fresh forcefully. Apparently, the more times Rainboramon decides to use attacks and his powers is when he digivolved to the Ultimate Level Goradramon. However, the DNA the newly digivolved Goradramon goes haywire and transforms into a full blown Yggdrasill, that for the moment, obeys all of Goradramon's commands, until Goradramon's unstable energy causes it to digivolve to Terranadramon. Having been cloned from a creature that had attempted to wipe out various species before and succeeded every time (INCLUDING humanity), Terranadramon goes to live on it's terrible legacy and destroy everything in sight.
As the four Digidestined are forced to flee the scene, they are teleported back home by an Azulongmon, but not the one they are familiar with. This Azulongmon, along with a Baihumon, Ebonwumon, Zhuqiaomon, came to this universe at a distress call from the still living Galacticmon, who was also busy informing the DigiDestined that they are in grave danger. The four sovereigns, intending on getting rid of Goradramon saying he's done far more damage than even they could even dream of repairing so easily, now have to worry about the destroyer of life itself, being the Terranadramon that now threatens this entire universe, as if Goradramon didn't threaten it enough already. Now feeling threatened by both them and his own creation, he flees the area again heading to where the DigiDestined are now, leaving the four Sovereigns alone with the Terranadramon. Even four Mega Level Digimon couldn't stand a chance against it's mighty power, as not only did it defeat them mercilessly, but their Data transformed into additional Terranadramon, so now 5 Terranadramon endanger the universe now!
The entire group arrives in one place for once after several months of having been apart, and for reasons relating to their own lives: Tai (22/24 years old now) was at his first diplomatic meeting, Matt (22/24 years old) was busy keeping Sora calm, who (Also 22/24 years old) is in no condition to help fight with her being pregnant with their first child at the time (To everyone else's surprise), Izzy (21/23 years old) was busy with college work, Davis (19/21 Years old) was not wanting to be around anyone after having broken Kari's (19/21 years old) heart after nearly killing T.K. (19/21), Yolei (20/21 years old) was busy trying to arrange her and Ken's wedding after Ken (19/21) proposed to her a few months prior, as Yolei wanted the whole group present, while Mimi (21/23) was possessed by Rainboramon for a while in a near useless attempt to lead Ken, Cody (17/18), and Joe (23/25) away from the group. Mimi was said to have been possessed long before Rainboramon even came anywhere near close to Earth at all, but others were unsure about this.
Suddenly, the meeting is cut short but what appears to be a Terranadramon, and with Agumon, Veemon, Patamon, and Gatomon being the only ones who could still digivolve, everything seemed hopeless at first, but they decided they'd go down fighting either way and warp digivolve right to Mega immediately. After a tough fight, the Terranadramon had already defeated Imperialdramon Paladin Mode, Seraphimon, and Magnadramon, leaving WarGreymon the only one left standing, and only because of the Dramon Killer weapons he had as well. Everyone is scared to death after two other Terranadramon show up, thinking this is the end, but Tai and WarGreymon refuse to give up even in such a hopeless situation as this. Tai's Digivice looked as if it were about to overload at that point, as WarGreymon suddenly digivolved into VictoryGreymon and knocked the two other Terranadramon out could. The main Terranadramon that was attacking them, however, survives the hit as if it were unaffected by it, as it is not a Terranadramon at all, but the very creature that all the Terranadramon was cloned from. Izzy was the only one who noticed prior to an arriving Goradramon announcing it, as the original creature had 6 large legs where Terranadramon only has 4 legs. The creature, dubbed Terranadrosus by Goradramon, had some kind of speaker attached to it's neck, and out of the bloom the big booming voice that is none other than The Grand One himself yells and scolds Goradramon for it's outrageous and unnecessary behavior. Goradramon tries to reason with him saying he only wants to stop the Terranadramon after the chaos he's only just now realized he's caused. The Grand One agrees to give him the ultimate power to do so, but he will lose practically all of his personality and emotions in the process. Goradramon just goes along not knowing what he meant, and suddenly he digivolves into Dragoramon, and just as a massive swarm of Terranadramon begin to surround them, the newly digivolved Dragoramon makes them all disappear along with all the damage they had caused.
Everyone's Digimon suddenly regain the ability to Digivolve after being energized by Dragoramon's aura, with Gabumon even digivolving right into ZeedGarurumon instead of MetalGarurumon from so much energy. They have a hard time trusting Dragoramon now after how all he did was wave his hand and the Terranadramon no longer existed in a way, but Dragoramon spoke nothing and merely glanced at them. Everyone is confused but it's lack of any reaction. It's personality erased, and it's ability to speak seemingly gone, Dragoramon stays absolutely silent. The Grand One's laughing is heard, saying that this is his punishment for having done so much damage to all the universes he's been to. The Galacticmon from before, now having digivolved into a Gaiamon, takes Dragoramon away back to their own dimension, with the last thing Dragoramon does before leaving along with Terranadrosus and Gaiamon is turning back time to 24 hours before the event, with none of them seeming to remember what had occurred.
T.K. wakes up hearing a voice again, also the voice of The Grand One yet again. T.k. wonders who it is at first but is then restored the memory of what had happened, with The Grand One saying he's impressed by how much everyone's changed in only 8 years even though The Grand One never got to physically see them himself. He tells him about how much their universe means to much to the Digimon living in his universe, and hopes he comes by to his own universe some time along with everyone else. The Grand One apologizes for all the chaos caused by Dragoramon, and leaves T.K. with a little gift he should not hesitate on using; An engagement ring forged from the same precious mineral that forms The Grand One's very armor itself, and even prettier than a diamond it was. T.K. doesn't know what else to say, but The Grand One suggests he go visit Kari right away. T.K. listens to The Grand One's advice and does just that, proposing the Kari with the ring he had been giving. Kari bursts into tears out of happiness, never having felt so happy to see him before after what Davis had done. T.K. thinks they should go to Yolei's wedding first though, since she's been going crazy about it to the point Ken was wanting to punch a wall out. Kari then suggests the same date would please both Yolei and her as well. Davis, having been given the suggestion to just apologize to Kari for what he did, heads to her apartment to do so until suddenly greeted with a hug from Kari. T.K., surprised to see Davis here, asks him to be his best man. Davis acts all speechless that him and Kari, but after what he did, they deserved to be together and so he agrees. When the wedding takes place, three special surprise guests are there attending the wedding; Both Gaiamon and Terranadrosus in human forms, and The Grand One himself in a rare moment of him not being in his powered up armor all the time.
The events that lead to the creation of Dragoramon lead to many universes being devastated in comparison to the Adventure 02 universe, which got off easy in comparison to the Xros Wars universe. The Xros Wars universe was nearly destroyed not by Dragoramon, but by a bunch of Terranadramon that apparently appeared and, as a result of appearing there at a certain time frame, erased all of the events of Young Hunters out of existence. The Grand One apparently didn't care as much about this universe as he did others. Other universes didn't suffer much at all other than the above stated side effects. About a few years later, a SuperMechadramon, among other B/C Universe Digimon, are found in this universe's Digital World.
Dragoramon, floating silently above Earth in the Adventure 02 Universe, thinks to himself as to much has changed over time, including the B/C universe. As the universes begin to restabilize after all the chaos that was caused by him, Dragoramon begins his age long duty of fixing the damage he was responsible for creating, but given the orders to leave this universe untouched. He traveled through many different universes during this chaos, perhaps every single known universe Digimon is known for having, perhaps some that don't even exist yet as well. Dragoramon, who normally feels no emotion, feels happy knowing The Grand One is happy for once, who he had normally viewed as some grumpy old man most of his life. Perhaps the rebellious Rainboramon was created for a reason, and Dragoramon wonders why, but that whatever the reason was, he was getting back to his jolly old self he once was when he was a lot younger. As years gone by, The Grand One continues his long beginning of work that lies ahead of him, and begins working on completely the massive universe he was inspired to create in the first place.