So I got tired of waiting for me to update my own comic enough to share spoilers, but I've been holding it in for like three years already so I'm taking the initiative and sharing it here!
This post includes details about the comic prologue, before the story, and the characters (and a little bonus section at the end, because why not)
Long-ass lore dump under the cut
--Prologue--
The Breaktale AU was originally a Genocide timeline, being messed with by a human with hacking capabilities. They did a regular Genocide run a few times, then started to mod the game. This led to the scene we see in the prologue, with the human killing Sans before where it was scripted in the game. This broke the game completely, with the next RESET solidifying how everything was messed up.
--Before the story--
In this new timeline, the story starts to differ from canon UT with the death of Chara and Asriel. Asgore, wracked with grief over losing his children, issued a decree that all humans that came to the underground would be killed. Toriel objected to this, and the two had a falling out, causing Toriel to leave. With his wife and children gone, Asgore ordered his royal scientist, Alphys, to begin researching ways to bring Asriel and Chara back from the dead, the practice of which was forbidden because of the price of such magics. The price being the cost of one life for another. Thinking that this was a small price to pay for bringing his children back, Asgore killed Alphys and exchanged her SOUL for his kids', giving them half a SOUL each. Asriel and Chara awoke with powers they had not had previously, due to Asriel still being half in his God of Hyperdeath form, and Chara sharing his SOUL. But Chara remembered something Asriel did not.
--Chara--
When Chara was brought back, they had a glimpse of canon UT. How the timeline was "supposed" to be. Chara saw how messed up their reality was and hated it. They vowed to "set things right" by erasing the timeline. Thus, they began plotting to get the six human SOULs that would fall, as well as the SOULs of every monster in the underground. Chara views everything in their timeline, including themself, as "wrong", and the only way they see to "fix" it is total destruction.
--Charcoal--
When the timeline reset from the original human's hacking, it was fractured far more than it is now. The story was fundamentally broken, and provided the basis for the "revised" Breaktale story. It only went from a shattered canon timeline to Breaktale through the intervention of Charcoal, an Ink!Sans variant. Charcoal had seen what was happening in this timeline and wanted to right it. After Sans was attacked in Snowdin and lost his arm and eye, Charcoal appeared to offer assistance. He explained alternate universes and timelines, and how Sans' timeline was fundamentally messed up. Charcoal could not make the timeline what it once was, but he could make a story around it, scripting events that had happened because of the damage to provide an in-universe reason for why they had happened. To do this, Charcoal had to focus on the particular story beats and how they could fit together. In alternate versions of what would become Breaktale, he got distracted by another AU, which explains why there are alternate versions of Breaktale.
--Sans--
Sans and Papyrus' parents died in the war between humans and monsters. Orphaned, Toriel took them in as the war was ending and set them up with a home in Snowdin. Life was relatively peaceful for the brothers, but their lives were interrupted when Sans was walking home one day when he saw Papyrus being picked on by a group of older monsters. He intervened, and a brawl ensued which cost Sans his arm and eye. This led to him meeting Charcoal. In the revised timeline, Sans is the only one to remember the previous one (contrasting with Chara, who only remembers the canon timeline). This secret weighs him down heavily, and he often feels alone because of the knowledge that his reality was rewritten. Tragically, because of their respective secrets, Sans and Chara were never able to bond over this.
Bonus Delarune stuff!!! vvv
--Alphys--
Before the events of the story proper, Alphys was doing research into alternate and artificial realities. She experimented with "creating" an alternate reality based loosely on her own. It never was quite clear whether she made the alternate reality or just discovered it... Alphys built two worlds for this reality, and populated it with monsters, many of whom were based on people she knew. These were her control variable, quite literally, in fact, as she could personally manipulate almost everything about the people and the places. But she needed an independent variable for her experiment. Alphys found a test subject, in the form of a human who had fallen into the underground. She codenamed them Kris and uploaded their consciousness into her reality to see how they would react in different scenarios. Think a kind of Truman Show mixed with XTale. Alphys also rewrote Kris' memories each time she altered something. But what was unexpected was that no matter what she did: memories she altered or people she changed, Kris would catch on that something wasn't right. It was puzzling, and Alphys was fascinated with figuring them out. Unfortunately, Alphys' experiment would be put on hold indefinitely when she was killed by the king. Despite her best efforts, it became one of her many unfinished projects... Also Alphys x Undyne was canon, then she died. Oops
