Sans AU Headcannons

Nightmare

-Nightmare is like a father figure to his teammates, because he's always there to make sure they're mentally stable, at the very least.

-Nightmare summoned Omni to make his mansion (not castle).

-Nightmare hates Dream because his ideals clash with his own, not to mention the fact that Dream didn't notice Nightmare getting bullied. He holds no other grudges though, and understands why Dream chose to fight against him.

-Nightmare would never and will never kill Dream. Torturing Dream is a job he leaves to his crews, and he generally doesn't fight Dream in battles, maybe the occasional tentacle stab or choking though.

-Nightmare slightly dislikes Error because he is the one being on his side that he has very little control over.

-Nightmare, while not liking him, holds no grudges against Ink.

-Nightmare is the kind to read you a bedtime story, then completely deny it later on.

-Nightmare sleeps with a replica of Dream's crown next him on his side table.

-Nightmare's tentacles immediately latch on to whoever touches them in his sleep (if he has them summoned) and will not let go until he wakes up.

-Nightmare can enter nightmares, but not dreams.

-Nightmare doesn't know of the existence of other multiverses because Ink and Error are scared that he will try to make whatever happened there happen in his own world.

-Nightmare can't end nightmares, but he can make them worse. He can...absorb some of the negativity from it though, so...slightly making it better?

-When Dream has a nightmare, Nightmare will be able to sense it, and he will go to Dream's aid right away. It doesn't matter what time of the day it is either. He will go to Dream and wake him up. Sometimes he talks to Dream about it, sometimes he just lets Dream hug him and cry. In the end, he will stay by Dream's side until Dream is awake again, reading a book or texting his crew in the meantime. He might even fall asleep with Dream, but that is very rare.

-Passive Nightmare is like Cross Chara, but he never speaks in public. He just follows Nightmare around.

-Water will melt Nightmare entirely. He'll just turn into a puddle of goop. He reforms eventually, but yeah.

-Nightmare knows nothing about how Dream was treated by the villagers. 

-Killer is his first recruit, and his right hand man (cough cough killermare cough cough).

-Nightmare's mansion has a game room filled with all kinds of arcade games, game consoles, and even 8 ball pool and poker tables.

-Nightmare makes money by looting the AU's he goes to of their cash. He gives 12% to everyone in the gang, 15% to him. The rest is used for mansion maintenance and necessities. 

-When Nightmare's drunk, he summons his tentacles and becomes extremely clingy and emotional.

-Nightmare loves whiskey, no chasers either. He just drinks it from the bottle, but doesn't drink often because, well, work.

-Nightmare's work isn't really signing papers, it's planning new strategies, logging AU's he's been to, etc.

-Nightmare never cries because the tears show part of his passive side. Extreme sadness or shock can also melt it away.

-Nightmare was once in the exact spot Ink was trying to teleport to, and he ended up having Ink partially teleport into his head. They were able to separate however.

-Nightmare has once tried seeing how many plates he could carry, tentacles included. He reached 77 before he fell over and shattered them all.

-Nightmare thinks Dream doesn't have to deal with stupid teammates.

-Nightmare hisses when faced with anything remotely holy.

-Nightmare used to really like seafood until he found his gang. Now whenever he eats it, he gets called a cannibal.

-Nightmare's sludge is either deathly cold or incredibly warm.

-Nightmare occasionally asks Ink to help him learn how to paint, since he finds it destresses him when books don't help.

-If Nightmare had enough negativity to feed off of, touching him would immediately make you wanna off yourself. 

-If you throw a bucket of water on Nightmare, he won't actually go passive because there isn't negativity or positivity to turn passive from. He just melts into this puddle of sludge.