In B/C, even though an unlockable cutscene shows the destruction of the Xros Wars Universe, there is a quest line titled The Great Battle of Xros Wars that involves restoring it. After obtaining Clockmon and Dragoramon, you can get the quest from Clockmon showing the time frame has been heavily damaged in one universe and is on the verge of destruction. However, apparently The Grand One is against repairing and does not send Dragoramon to simply fix the damage... It is without a doubt the hardest quest line in the entire game, even harder than The Grand Trial, which must be completed before being able to get this quest at all. Some similar late-game boss rush quests exist, but none as severe as this one, as it demonstrates the more godly power that The Grand One is meant to have when in his own universe. However, as the entire quest takes place in the damaged Xros Wars universe, one wonders how he was even able to control these Digimon except for the last three creations.
The Quest (MAJOR SPOILERS)
With all other damages fixed in every other universe, such as the restoration of the timelines to their proper order all except the Adventure 02 universe, The Grand One tried to hide the destruction caused in the Xros Wars universe by having Dragoramon fake it having been fixed. An unnamed tamer, however, obtains Dragoramon and learns of this from a Clockmon also owned by the tamer. Knowing this is top secret information, the player secretly makes his/her way to the Xros Wars universe and begin by at first what appeared to be merely restoring the DigiQuartz. However, a possessed Mechadramon Team arrives soon after, and at first Clockmon reasons that they can't DigiXros in the DigiQuartz. However, hopes for an easy get away are shattered when it proves them wrong by becoming SuperMechadramon, and the first boss of the quest is faced. Quartzmon is not anywhere present at all, neither is the DigiQuartz, as both have been destroyed along with several main cast characters. Clockmon reveals itself to have the memories of another Clockmon. Suddenly a GrandGeneramon appears, as a remnant of Quartzmon's abilities.
The next part of the quest reveals, however, the DigiQuartz wasn't the only thing damaged to the timeline, and that several main characters are deceased for unknown reasons. The second and final part of the quest begins when the Death Generals are revived for reasons currently unknown. However, it soon becomes clear that The Grand One has discovered the player trying to fix this timeline and tries to stop them. Forcing the Death Generals to become GranGeneramon even in the DigiQuartz zone, the next boss battle begins. Get used to the boss battles, as you'll see nothing but this and cutscenes for the rest of the quest, with no chance to heal inbetween.
After GrandGeneramon is defeated, an Arclimon shows up saying this has gone on far enough. At first, an Arclimon may not seem that imposing, but it reveals it's digivolved form Arclimon FM and the next boss battle begins. This Arclimon has Gaiamon's Overdrive as a trait, clearly another attempt at The Grand One to stop you, giving it the power level of Terranadramon. Due to this and the Arclimon Line's signature ability, this can be one heck of a fight and possibly be even harder than merely facing Gaiamon. Speaking of the devil itself, guess who is next almost immediately after Arclimon FM.
At first a team of three of the toughest Digimon in the game seems odd given their size ranks, but these specific ones are size rank 1 for each, meaning each of them takes up only one zone on the field. It is impossible to KO Galacticmon without KOing Armageddemon and Ogudomon first, who then uses Ragnarok Cannon to leave you at 1 HP and Zero MP for ALL Digimon in your party. Galacticmon's HP is restored to full health each time it uses Ragnarok Cannon as well. The pain doesn't end here, and The Grand One continues to show his stubborn nature he's always known for. Galacticmon digivolves into Gaiamon and right into Armor Mode, while doing damage to your team at the same time.
You may have faced Gaiamon already at this point, but he wasn't in Armor Mode when you face him as part of The Grand Trial. Armor Mode effectively quarters all damage received and makes it immune to status effects. If it wasn't mentioned already, ALL of the bosses faced in this quest have 99,999 HP and MP and 9,999 Attack, Defense, Spirit, Wisdom, and Speed, and are at Level 999. Be careful, as ALL of Gaiamon's attacks hit all five zones due to it's massive size rank. Clearly The Grand One understands game mechanics very well and is trying to wear you out as much as he can. If you just so manage to survive the horror of this, you'll freak at the horror seen next.
The quest namer makes it's only boss appearance if the version isn't Jade, and it's actually HARDER than the Jade Version Terranadramon fight. The Grand One has stated you have gone too far as Clockmon leaves with Dragoramon to fix the timeline of this universe back in the past, so decides to bring the past to the present by bring all the Terranadramon from the past in this universe into the present. Clockmon and Dragoramon only had enough time to get rid of a majority of the Terranadramon, and you have to face ONE THOUSAND Terranadramon with five at a time to get any further. Think it's over after this? It isn't as the Gaiamon reappears and uses it's own version of Ragnarok Cannon to leave all but one Digimon in your party with the fainted status and have the surviving Digimon be not only at 1 HP with 0 MP, but also a sharp nerf to Defense and Spirit. After The Grand One remarks "We're not done yet!", he unleashes the horror of the most powerful lifeform onto you immediately after Gaiamon uses Gaia Cannon on your Digimon.
The timeline restored, but one threat remains, and that is the threat of the real deal instead of a just a mere replica relative of it's mighty power; The almighty Terranadrosus! And this time, The Grand One decided this situation was SO urgent to stop you he sent her two relatives to help her out. There is only one of each, but that's all there needs to be: Terranadrosus has double it's 9,999 Attack stat as well as a boost to speed to make sure it can get in several moves before you can even get a chance to heal. Xilatealeon has double it's 9,999 Spirit and a similar but slightly smaller speed boost. Shieldrus has Infinite Defense and Wisdom and can heal the other two easily. Your only savior here is that you get the first move, and you better have items left even after this massive onslaught. Terranadramon heals HP every time it gets a chance to attack, and also has a clearly hacked 999,999 HP which isn't possible to obtain as a playable Digimon. Shieldrus can revive the other two, so focus on Shieldrus first even if it means dealing only 1 Damage each hit, so unless you have moves that deal a set amount of damage Shieldrus will just wall you to kingdom come. This specific boss battle has it's own unique battle music specifically for this deadly encounter. Only until Shieldrus is KO'd can you even dare focus on defeating the other two. Shieldrus, fortunately, has only 9,999 HP compared to above mentioned Terranadrosus's HP and Xilatealeon's 99,999 HP. Xilatealeon has an equivalent to Ragnarok Cannon is uses every 7 turns of it's own turns. Shieldrus, having Infinite Defense and Wisdom, and Fracktamon, having Infinite Attack and Spirit, causes one of the possible Irresistible force paradoxes in the game, but nowhere near as severe as the intentional Argendramon and Giganodramon rivalry. Speaking of Fracktamon, due to it's signature ability protecting it from damage, and provided you keep the Speed stat at an all time low within the 100s, Fracktamon can be very useful in this boss rush provided you have the patience. Using Fracktamon, however, will not earn you any PSN Trophies at all, just like with using Fracktamon in other high rank situations as well. Terranadrosus will most likely be the last to fall due to it's stats, and after the entire team's defeat The Grand One will be impressed so much he'll just let you go free of charge, deciding that having to face all three is a proof of dedication to the game.
Xilatealeon, Terranadrosus, and Shieldrus are the only boss opponents not obtainable in the game, as they are not related to the Digimon franchise at all. Instead, however, they have Digimon designed in their own image. These three typically only appear when the situation is very severe, and serve as The Grand One's most powerful creations. Xilatealeon acts as a Cannon (High Spirit), Terranadrosus acts as a Spear (High Attack), and Shieldrus acts as a Shield (Infinite Defense and Wisdom) in terms of their stats.
As the Xros Wars timeline is now restored, many Xros Wars related quests can begin now that this crisis is over. The Grand One will use Gaiamon Armor Mode against you if faced again, and will have the HP stat of Terranadrosus as well. Scan data of Xilatealeon and Shieldrus's Digimon Counterpart are unlocked as prizes, and the X-Antibody can now be obtained in quests. After the serious boss fights you've been through, you must wonder what The Grand One himself is truly capable of and why he went through all that hard effort to try and stop you... A special cutscene version of the entire quest is unlocked, showing somewhat different events going on, such as the involvement of the Xros Wars cast in the battle.