a dreamtale au (words colour-coded for easier reading)
Nightmare eats the apples and the tree dies, Dream gets turned to stone, and Nightmare goes out into the multiverse, hundreds years later Dream emurges and goes looking for his brother.
the big difference? Nightmare is the doe eyed optimist and Dream is the shut in cynic. Nightmare insists upon playing with the villiagers, despite how much they hate him, they only play with him because Dream threatens to leave and take his powers with him. Dream would 100% leave the tree for dead, nothing is more important to him than his brother, if only Nightmare could see that they only stand him to get Dream's positivity.
so the apple incident. Nightmare is chilling under the tree, Dream went off foraging, everyone is sick of this little kid that brings negativity everywhere he goes annoying them, so they plan on getting rid of him and kidnapping Dream. Nightmare begs and pleads for mercy and kindness, he calls for help but nobody came. so he did the one thing he wasn't allowed to do; he ate the apples. Dream comes just too late, finding his little brother with black stuff pooring out of his eye sockets. Nightmare's corruption furthers, tentacles ensue, and people either attempt to run, or attack, he shouts for them to leave him alone, his power reaches out and kills everyone around him, except Dream. Dream isn't scared of his brother, he is scared FOR him. he attempts to approach him and calm him, but Nightmare starts crying from the pain, how overwhelming it all is, or from what he's done dream doesn't know. but Nightmare screams for him to stay away so he wouldn't hurt his brother too, but all that does is start the petrification. Nightmare desperately tries to stop it, but his efforts mean nothing. all Dream does is reassure him, telling them it would all be okay. as he is fully turned to stone his eyes are full of tears but there is a smile on his face, Nightmare is the only one who ever got to see it.
after his brother was petrified, Nightmare sat at the dead trunk of the tree and cried for a bit, wanting to get away. and he accidentally opened a portal to killer's AU. (this was cause killer was feeling the same thing deep down; wanting to escape his empty home and be free of his sins) so he HELPS killer and promises to help him and look after him like Dream did. over the time of Dream being stone, he recruits the gang (killer, horror, dust, cross WHO IS VERY IMPORTANT, and kinda error) and ink, blue, red, and core are trying to stop them cause they are spreading negativity and harassing AU residents. Nightmare only needs the general balance to be maintained, but sometimes he doesn't tell them that there is too much negativity cause he hopes that if there is more positivity Dream will wake up again and he can apologise for all he's done (he hasn't done anything wrong) and be with his brother again. the gang are like his little brothers, he learns how Dream felt when they were kids. he has to tell them how to care for themselves, not kill each other, not destroy the castle, and to leave the au residents alone (they won't, they think it's fun) the castle still exists, they found an au that error had cleaned out of residents, but ink stopped him from totally destroying, and Nightmare created them a home where they would all be safe, happy, accepted and loved.
Ink and core where there when Dream woke up. Dream imediately was on the offensive, questioning who they were and where is brother was. they told him their version of the story, how he was terrorising the multiverse and evil. dream did not believe this for a second. the last golden apple was at his feet, it had been there for centuries, Nightmare refused to touch it as he had killed all the others and hoped its proximity to dream would allow it to last. Dream took and ate the apple, seeing no point in keeping it since their world was already dead. he gained his basic powers, altough his little stick is much more of a bo staff. Dream was dragged to the omega timeline and begged to help them stop his brother, he remained cold and refused to do anything they said, he only broke once he realised if he joins them he can find his brother easier, he didn't care if his brother MAGICALLY decided to take over the entire multiverse, his brother deserved it for all the shit he went through. he tolerated core, grew to find ink endearingly chaotic, (he even got Dream to chuckle a few times) and he appricated blue and his optimism, it reminded him of the brother he was desperately missing. Dream basically used blue as a replacement Nightmare for all of the nights they spent apart, he smiled in blue's precence sometimes, accepted and gave him physical affection, and called him affectionate nicknames.
when the two finally do meet on the battle field, Dream is relieved to see his brother is alive and well, but Nightmare is TERRIFIED. he thinks Dream is here to hurt him for everything since he's with the people who have been attacking him and his friends. (Although they wouldn't attack if his pals actually listened to him for once) Nightmare keeps running, anytime he sees Dream on the stars' side, he runs back home and ends up hiding in his corner in the library and sobbing. he feels so guilty and sad and scared. meanwhile Dream is distressed that he keeps failing to reach his brother in time. Dream trains to be more athletic in hopes of catching his brother before he disappears into his own world again. he also blames the stars for his brother fleeing at every interaction.
once they do talk, it's a lot of crying and Dream gets to finally hug his brother again and reassure him he's there for him and always loved him, and would never hurt him. it's kinda like a spin on the entire multiverse knowing Nightmare is gone and Dream is in denial, instead everyone else is just ignoring what Nightmare has to say and profiling him like the villagers did, and Dream is the only one who actually wants to listen and help his brother feel better.
perhaps Dream would be a couple/few years older than Nightmare too.
might make this a proper fanfic, wish i could draw cause i would love to make art of these boys.
(@dreemurr-skelememer I originally tried to send this as an ask to you since i know you like dreamtale and it's aus a lot and thought that you would like to see it)
EDIT: HERE IS THE LINK TO THE FIC I AM UPDATING IT AS FAST AS I CAN https://archiveofourown.org/works/46784704 if you are interested in this AU stick around for the fic please