Undertale: Boss Rush is an Undertale Alternate Timeline (or AT) created by Trackrays. The name of the AT is still subject to change, if the creator can find a better name than what he thought of.
Backstory
It was an ordinary Genocide route up until Snowdin - ever since the whoopee-cushion encounter, Sans observes the Human and sees them kill every monster. He still gives them another chance, however, and does not do anything. But, as soon as the Human strikes down Papyrus, Sans realizes the Human had no intention of stopping and rushes away from Snowdin (not in the direction of the Ruins, as, given from the Human's killing spree in Snowdin, it's clear that they did the same to everyone in the Ruins). Seeing as the next area is Waterfall, he manages to find Undyne and tells her the story before asking her to help evacuate the monsters.
As Undyne works on evacuation of Waterfall's residents while Sans rushes to Hotland to inform Alphys, the Human arrives at Waterfall as Undyne's evacuated around 60% of the monsters. She battles the Human to hold them off as the rest leave - however, after a decently lengthy battle, Monster Kid attempts to help Undyne just as the Human is about to land a final blow on the latter, resulting in Monster Kid's death. The Human, rushed to kill the other monsters who attempted to evacuate as Undyne, being too weak to help anymore, tries to rush to Hotland to get help from Alphys = carrying the guilt of the monsters she ditched along with her.
Thanks to the combined efforts of Sans, Undyne, and Alphys, the monsters from New Home and Hotland get evacuated safely, Undyne gets healed, and Sans and Undyne go rush to Asgore to get help. However, Mettaton being the self-absorbed robot he is, didn't pay attention to the evacuation announcement by Alphys and the personal request to evacuate from Undyne, as he had a show coming up. Therefore, he meets the Human in person and, thinking they came for the show, gets into his EX form to put on a good show. After a relatively short battle, near the end of which he realizes his mistake in assuming the Human was relatively harmless, he gets killed.
Alphys, already depressed from Undyne losing, monsters getting killed and whatnot, upon finding out Mettaton died (from the Human's body camera, which was present in UT, again), gets the Amalgamates from the True Lab and attempts to stave off the Human in Hotland as the other boss monsters regroup in the Throne Room, seeing as Asgore isn't likely to move from there.
After a somewhat successful battle for Alphys (as the Amalgamates were at the front, tanking and dealing damage), Alphys gets cocky and tries to fight the Human herself, only to get instantly killed in one hit. After that, the battle against the Amalgamates continues, until said Amalgamates get tired enough for the Human to flee, as the latter dashes past them and proceeds.
The Human proceeds through the usual route, reaching the Judgement Hall (as Asgore did not want to fight in the Throne Room for fear of ruining his flowers) where the Human faces Sans, Undyne, and Asgore (with a sneaky Flowey watching the battle while remaining unseen). As soon as the battle starts, Asgore whips his trident out while the other two similarly get ready for battle - however, as Sans made the elementary mistake of standing on Asgore's right, he gets slashed by the aforementioned trident, resulting in his death too. Undyne and Asgore squabble as the Human takes this chance to slice Undyne with all their power, damaging her enough to send her to Undyne the Undying.
The final battle begins against Asgore and Undyne the Undying (being Asgore, Asgore shatters the MERCY button soon after Sans' death), with Sans watching for some time before dying midway. Right after Sans dies, Flowey chooses to absorb Sans' soul before it shatters, giving him enough strength to pose a somewhat notable threat (as Sans, despite being physically weak, can simultaneously dodge and use a lot of attacks in a fight, meaning he has a lot of mana/stamina). He then chooses to join Asgore and Undyne out of his worry of dying at the Human's hand and in order to regain control over the timeline, as the most determined creature. The battle goes on, with Asgore fighting the hardest for his fallen subjects, his wife, and everything else he's lost to humanity - Undyne, meanwhile, slowly starts fighting less vigorously out of exhaustion and depression (the latter more than the former), and Flowey, much weaker in general than the other two, being unable to fight at the same level as them.
Soon enough, Flowey bites the dust as Undyne starts melting, resulting in Asgore fighting more or less alone. Although both Undyne and Asgore attempted to, neither of them could leave the Human to get at least one of the Human souls as it resulted in them getting damaged even further, even with their speed. After more fighting, Undyne dies, resulting in Asgore standing alone.
Asgore, like Flowey, attempts to absorb Undyne's soul but was too late. Knowing he has no other option left, he fights with all he has, resulting in him starting to sweat as well. Some fighting later, Asgore eventually drops his trident and falls to his knees as the Human, driven by Chara's impatience to reset, blindly cuts Asgore's left arm and moves on, leaving him nearly dead. Mustering his remaining strength, Asgore chucks the trident as it lands directly in the Human's chest, shattering their soul for good. Asgore, having no more energy, passes out as the Underground is left nearly empty - the majority of its population evacuated, the rest dead.
Characters
Sans
The ex-Royal Judge, Sans chose to break his routine and take action from the time the Human entered Snowdin (arguably a bit too late, as he let Papyrus die), instead of waiting till they reached the Judgement Hall. However, Sans did not get to see the light of day when the Human was no more, as he died at the start of the battle itself to Asgore's trident.
Asgore
King of Monsters and/or the Mountain King, Asgore eventually managed to let go of his past losses to prevent losing everything he has left, including his own life. Although he could not save his wife, Toriel, Alphys, Sans, Undyne, his son (Flowey - although, he did not know Flowey was Asriel reincarnated), or some of his people, he managed to ensure that monsterkind could live to see another day.
Trivia
- Sans was removed from the rest of the story via death for two reasons:
- To ensure that the AT did not become like the many others (ex: Ainavol, The Bonely One, Call of The Void, etc.)
- As a reference to a meme.
- As the creator is terrible at making names, Undertale: Boss Rush was named so due to the "boss" monsters gathering to fight the human (Sans, Undyne, Mettaton, Flowey, and Asgore - had Papyrus and Toriel joined, the AT would have been fully true to its name.)
- The main reason Flowey chose to side with Asgore and Undyne was to avoid death at the Human's hand, as they knew the Human was powerful enough to end them nearly instantly without any SOULs.