Fan:Zerosumon

Over 2,000 years, one feels absolute suffering. Duo, heartbroken at the death of the one she held most dearest to her, is approached by Commander Kiryu. Kiryu, not even caring about her chosen soul mate at all, approaches Duo calmly, but her only response is absolute rage with how he chose to do nothing to save him. Duo, having lost all respect for Commander Kiryu and lost all reason to live, decides that if she is to suffer alone for the rest of her life, she wants him dead. Thus, with sheer force Kiryu is thrown through a wall, but he survives. Just as SuperMechadramon appears to stop Duo, Duo forcefully beats SuperMechadramon and destroys the six DigiEggs that spawn from it's broken remains, permanently destroying it. Kiryu, pleading to Duo that it was all the fault of a Digimon called Zerosumon, even pleading to her that his son will have nowhere to go if she kills him. His son, however, has no respect to him anymore finally knowing what Kiryu has been doing over the years, and even no longer considers him his father if all he tries to do is find a mother for him. Kiryu, shocked, is then impaled brutally by Duo, who then bursts into tears and flies off to some grave stone. Back in the present, Duo wakes up from this horrible nightmare, revealing that all to be a terrible nightmare. Duo, having grown soft over the time she's spent with Marcus, tries to figure out what this nightmare means and why she keeps having it. Zerosumon, whatever Digimon it is, managed to kill everybody except Duo, Kiryu, and some others in the future. It's power, surpassing that of Terranadramon, makes it an extreme threat to everything around it. All of reality breaks and shatters when it moves, and every attack from it can kill off hoards of Digimon Super Ultimate Level or lower in an instant, and when it kills them this way, it is permanent. Even Dragoramon cannot undo the actions of this Digimon, or what Duo calls an absolute monster, not believing it even has it's own soul at all. Just what is Zerosumon anyways?

Digimon: Bursting Cries: Zero Finale
The series finale movie to Digimon: Bursting Cries is it's only major appearance other than B/C obviously. In an alternate future occurring after the events of Bursting Cries, almost the entire multiverse is destroyed with humanity extinct in nearly every one of them except for the B/C universe. Several villains became honored as heroes by those who did survive Zerosumon's onslaught, but Duo was the most suffering of the ones who survived. She only survived because of what she really was, and on the day she was to be married to Marcus, the day she thought would end all her days of regret and pain, was turned into the worst day of her life when Zerosumon suddenly appeared and killed nearly everyone, the only survivors on the Digital World at that time being Commander Kiryu, Duo, SuperMechadramon, Kiryu's son Drek, where everything else was slain by Zerosumon, even the god-like Dragoramon could neither protect anyone or even protect itself against it. The Grand One was nowhere to be seen, so Old Gaia couldn't be summoned to stop it, and everything was over in what appeared to be an instant. Duo wakes up and sees Marcus having been one of the victims of Zerosumon, with surprisingly, as much as the unstoppable force Zerosumon was, is found laying dead not far away from there. Marcus's Agumon arrives from their dimension, and many surviving tamers and Digimon throughout the multiverse feel absolute sorrow because of the surprise incident that occurred, calling it far worse than Terranadramon.

Duo, having been unable to witness the exact event as she was knocked unconscious by Dragoramon and Arclimon, both Digimon owned by Duo since the very beginning of the season, in-order to protect her as they're watching helplessly as even the Super Ultimate Theridramon is being mauled by by Zerosumon as even the ultimate power of light itself wasn't enough to destroy Zerosumon. Soon after, Theridramon perished at Zerosumon's power, and Zerosumon, with Dragoramon teleporting Duo away to The Grand One's house knowing it'll be the last move he makes and knows that is the only place to keep her safe, as Zerosumon destroys both him and Arclimon next. With no Dragoramon able to stop them, the Terranadramon go berserk from their containment, but also fall victim to Zerosumon's onslaught, with only Old Gaia and SuperMechadramon remaining to stop it. Old Gaia, however, was knocked into stasis by something, as well as The Grand One himself, and thus were unable to do anything to stop the rampaging monster. What was a day of happiness in the Adventure 02 universe became a day of horror as Zerosumon slaughtered both humans and Digimon alike in an unstoppable rage, and some even went as far as to resurrect old villains such as Lucemon Shadowlord Mode inorder to stop it, but all was hopeless. Zerosumon dies soon after nearly everything else had, and not even from wounds at all, as it appeared completely unscratched as nothing could even get close to it. Marcus, it's final victim, was what caused it to suddenly die for no apparent reason. After limping away for some reason, it collapses and perishes, not reverting to data at all, and lays there as a physical body now lifeless.

A month would pass during this rampage, and the damage felt irreversible, with Duo awakening in panic wondering about Marcus, not having realized it's been a month since she was knocked out. Duo feels sudden sorrow despite having not figured out what happened, and heads back to the Digital World to find everyone nearly slaughtered and dead, even her beloved Marcus, and as soon as she saw him dead, she was suddenly heartbroken, as everything she had up to that point in her life was gone, and even worse, Dragoramon wasn't alive anymore to fix all the damage, like it even could anyways. Commander Kiryu, seeing Duo in her sorrow, makes one final move on her saying he'd comfort her, but soon his son came bursting out of his friend's house, who are now dead because of Zerosumon, screaming it was all Commander Kiryu's fault in absolute misery. Duo, suddenly feeling absolute rage, slowly approaches Kiryu and shouts and swears at him for what he had caused, even to the point his own son was bursting out crying and was apparently tied up somewhere which led him unable to warn his friends nor Duo about what Kiryu was planning: He wanted Marcus dead the whole time so he could have Duo to himself. Duo doesn't even know what exactly what was going on, but ends up killing Kiryu then walks back over to Marcus's body crying. Even someone so tough like her never felt such sorrow before...

However, all of the above was actually a horrible nightmare for Duo, who wakes up screaming. She moves to the other bedroom to see everyone else still alive, even Marcus, and gives a grateful sigh of relief that it was only a nightmare... The first nightmare out of many that will signal her future... But she doesn't know how Zerosumon was created even though it was apparently Commander Kiryu's fault. What happens next is to be revealed at some point later....

Digimon World: Bronze and Cobalt Versions
Zerosumon's area is unlocked provided Goddreamon, Theridramon SU, and Dragoramon are present in the DigiFarm. You also must have completed The Great Battle of Xros Wars questline in-order to unlock the area as well. The area is about 2 years into the future. Corpses and bodies lying everywhere, with entire towns and cities destroyed and damaged. X-Antibody Digimon are common enemies here, and so is Mecha Rogue, which are at higher levels than the ones fought in The Grand Trial area. Eventually leading to a clearing where Duo, the only appearance of her anime incarnation in-game, is mourning over the loss of Marcus, but she soon disappears in an illusion. Zerosumon's roar is heard in the distance, which Duo leads the path to Zerosumon before disappearing. Zerosumon, being one stage higher than Terranadrosus and Terranadramon, is the toughest Digimon in terms of level. However, if your Digimon's speed is high enough to where you move first, however, Zerosumon's signature trait activates, which transforms it into a Burst Mode-like state which makes it move first instead. In this form it has double it's Attack and Spirit, which are both at 9,999. It's stats are also that value as well, with HP and MP also being at max. It's move strategy is similar to facing Terranadrosus, Xilatealeon, and Shieldrus all in one Digimon, but it's signature trait, Chosen One, makes it all out offensive based when it's activated. Either way, it is of best interest to let Zerosumon move first by having all your Digimon's speed stats be low enough to give it the first attack but also high enough to react to it right away. It's signature move can one hit KO anything, and has no drawbacks either, as well as minimal MP usage, but Zerosumon rarely uses it normally when faced as the ultimate boss. Another move it learns allows it to recover 75% of it's max HP and MP in an instant upon using it. It's move usage, other than it's signature move, is completely random, but it's worth noting that after a certain amount of turns Chosen One wears off and leaves Zerosumon weakened enough to KO provided you can hit it hard enough. Zerosumon dies after being faced, leaving only it's scan data behind. When DigiConverted, all of it's stats are at their lowest possible interval, and Zerosumon levels up the slowest of any Digimon due to it's high level. Zerosumon is banned from even modes that allow Terranadramon, as most of it's attacks deal damage high enough to OHKO everything.