About the AU
The first part of the AU, known too as Blind Art, tells Anti!Ink's story, while the second one, Blind Joy, follows Nega!Dream's.
In Blind Art, Ink lost a fateful fight against Error, which ended with his soul destroyed. Isolating himself, he began to forget his own meaning and deem himself useless until Dream found him and tried to help him. However, everything went wrong when Error found his timeline, causing a chain of chaotic events that ended with the destruction of more than half of the multiverse, Ink's death, and Nightmare taking advantage. This part has two endings.
Blind Joy tells the aftermath of Error and Nightmare's influence in the multiverse. Out of vengeance and hatred, the latter put an end to his one-sided rivalry against his brother by taking his emotions as well as everyone else's, leaving Dream as a suicidal, nervous wreck who can't feel anything but self-hatred, fear and despair. As opposed to Dream's successful attempts at trying to save Ink, CORE!Frisk's attempts to save him don't go as swimmingly and cause Nightmare to harm Dream further as a threat. Ink still makes an "appearance" here, though as his essence. Unlike Blind Art, this part only has one ending.
The main characters are Ink, Dream, Error, Nightmare, CORE!Frisk, US!Papyrus and Sans and Science!Sans. Minor characters are Geno, Fresh, UF!Frisk and Flowey and SS!Chara, though they aren't as relevant in Blind Joy.
In Blind Art:
Dream!Sans: he is heavily protective of Ink and does everything he can to assure that he regains his true self. However, his attempts fail, and after a fight against Error and Nightmare, he turns to be the one in need of help. The battle against Error left him with a broken femur, a crack on the side of his face and a gash across his midsection, and he escaped the fight against Nightmare with heavily broken ribs. He is noticeably weaker, as a large portion of the multiverse had been destroyed.
Error!Sans: his persona doesn't change until the end, where he loses what was left of his sanity after his LV raised beyond the limit. His raised stats gave him stronger powers, such as being able to use his strings as whips or use them to grapple others and electrocute them. After the fight against Nightmare, he was left with a crack running down the bottom of his left eye and a damaged leg. After Nightmare left, he tried to finish the fight against the others, but he was finally defeated by Ink, who managed to summon one last, powerful attack through Determination.
Swap!Papyrus: he is shown to be more overprotective of his brother, and everyone left alive in general. He's somewhat anxious and shows slight signs of paranoia, and can be rather aggressive at times. He's heavily grateful to Ink for having saved his brother, and does everything he can to help him until he dies.
Geno, UF!Frisk and Flowey, Fresh and SS!Chara: they only have a notable role in this part, where they help in the fight against both Error and Nightmare. Geno joins the fight driven by empathy, while Fresh joins because he doesn't want Error to end the multiverse, as that would eventually kill him.
Nightmare: he's the one to stop Error from killing Ink and Dream, as he needed the latter alive. He warns his brother of his new powers, such as being able to absorb and corrupt anyone's feelings if they are weak or wounded. He eventually leaves, temporarily, as the others managed to weaken him through light magic, though not before stating that "everything was far from over; in fact, it had just begun."
In Blind Joy:
Error!Sans: he is depicted as far more unpredictable, twitchy and vengeful than usual, as well as being heavily paranoid and sometimes psychotic. He doesn't try to destroy any other AU in fear Nightmare would target him for it, and mostly stays in the anti-void. He's the first to be mind-controlled by Nightmare, and is eventually killed by him.
Swap!Papyrus: he has a slightly bigger role, though he doesn't do much aside from making sure the survivors are alright and find ways to help Dream alongisde his brother.
Blueberry: he has a major role in this part as he is the one who is truly willing to help Dream and the one to find clues as to how he can be helped along with Science!Sans.
Science!Sans: he is somewhat more anxious than usual, knowing what is happening to the multiverse. He agrees to help Dream along with Core, Papyrus and Sans, though he ends up getting killed when he accidentally stumbles upon Nightmare's dimension while he was trying to aid a comatose Dream.
Geno, UF!Frisk and Flowey, Fresh and SS!Chara: they are very minor characters; all of them end up losing their emotions and becoming paranoid and aggressive after Nightmare's raid, except for Fresh who is emotionless.
Nightmare: he takes over what is left of the multiverse, though he leaves Core, Blueberry, Papyrus and Science!Sans alive and unharmed because he finds their hopes to be amusing. He keeps gaining new powers, such as mind-control or the ability to take depressed people's memories. He constantly taunts and hurts Dream to keep his friends from trying to help him.
Ink!Sans: he regains his true persona after death, though he is not a corporeal being anymore. He can manipulate certain aspects of the Doodle Sphere, such as guiding people by blowing loose pages around or creating rain. He is aware of everything that happens inside the timeline, though he can't always manifest himself.
Ink never lost his soul prior to the events of the AU, so his backstory may differ from canon. His personality is also slightly different, though it doesn't vary much.
Even though Dream was redesigned, he retains his original design throughout the AU.
The story takes place after (or in-between?) the AskError comic and perhaps in-bewteen the unfinished Dreamtale comic.
The Doodle Sphere in this AU is much different than what it looks like in canon. Here, it's portrayed as an endless field made out of sketchy grass, with the sky being a constant sunset with loose papers floating around. The AUs are doors, painted over to represent each universe accordingly - they can only be damaged with magic, and if the doors are destroyed, the respective universes are gone for good. After Error's attack, the sky becomes a purple/red blob and the grass turns grey.