Everything begins, before the last human falls, Alphys discovers that with the power of a human soul two monsters could fused, she notifies this to Asgore and he leaves her the full access to the human souls, when the player starts the genocidal route, it begins to notice the changes, since at the time that the player damages Papyrus, Undyne arrives in time to take both sides of Papyrus before he turn into dust, she manages reach quickly the laboratory asking for immediate help to Alphys, she recalling the discovery, she quickly went looking a human soul being this yellow, being the justice, She quickly asks Undyne to grab the Papyrus’s soul and hers with the human soul to save him, in doing so their bodies merged together, into a extremely strong being, now having 3 souls: Papyrus, Undyne and the human soul, their physical features were more like Undyne, this is because the soul of Papyrus is badly damaged, Alphys was amazed, and about to start asking questions, he just disappeared, it left her completely stunned, Alphys didn’t know what to do, the most powerful warrior and Sans’s younger brother, had simply gone away, while the genocidal elapses, the player notes that it doesn't have meetings with Undyne, everything goes "normal" until "kill" to Sans, when the player is removed from the room, Alphys comes to succor Sans quickly before he turn into dust, doubtful of what she would do with the human soul of aquamarine color that means kindness, she put together the 3 souls, and the physical forms become one, retaining the 3 souls, comes out a being shaped of Alphys with the difference that in his forehead have the mark of a bone, and his clothes were Sans's only with lighter shades, he name himself Alsans, he had clear his mission, get rid of that human once and for all, he ran to the throne room only to find Asgore's death, the human only turned on his heel to find a look of hatred, the battle was not very long, the human falls pretty fast, but it meant the sentence, because he had no reset possibility, or even start another game even if was pacifist with this world that was just... desolate, Alsans went in search of the remaining souls, but only had three because the purple one disappeared, he downplayed this and take them, eventually he learned to control his power and until at finally was able to get out of that universe through a Gaster blaster. In a vacuum, he observed that he could go to other worlds, it attracted him a lot and from there, he started his research on all what he could.
Easy, he has a "small" house, if you've never been there, it is quite difficult to find, and it has a library, where have all the information gathered from other Au's that he has visited.
Why they are more like Alphys and Undyne if it is assumed that their physical form also merge to create an entirely new being? Well, this happens because the physical forms of Papyrus and Sans was in a critical state, where they were badly wounded and were to become dust, this causes the physical form of them do not have much effect on the other to perform a full combination, leading to more closely resemble them.
Papyne got there by chance, seeing Alsans he did not recognize him, but Alsans recognized him, he explain everything that had happened, eventually he had to accept it, but Papyne has always maintained that so explosive attitude and that smile. About Gaster, he followed Alsans, but without being noticed, when Papyne arrives, he was glad but was more scared, because he was afraid of how they will react, so show himself with the kids first, and then finally, with his sons which reacted with a hug because his father had been able to return.
