Negatale is an AU where the monsters have negative effects, negative colors (the only common color in this AU is blue, like how in Underfell, it's all red), and negative emotions (sadness, depression, grief, regret). Their personalities are same as in Undertale (the original) but the difference is their emotions.
Information about the AU
Negatale is like Undertale but in an opposite or negative way. For example, Sans being all happy in True Pacifist, while he is sad and angry in the Genocide.
Sans and Papyrus are still related to each other, but Sans cares about Frisk too much. He thinks Frisk is his safety or a shield. Sans is mostly scared of Chara even though Chara loves him.
Papyrus is still the same, except his cooking and his emotions. He still wears his battle body, but his scarf does not go up in the air. As his scarf is in the front, if there was wind it would fly in his face. Papyrus mostly can't see what's through his scarf when it is in his face.
Grillby is an opposite of the original Grillby. He is cold-hearted and is made of ice since Negatale is all blue (like Underfell is all red).
Negatale is not just characters with negative emotions, they still have feelings (like happiness). Every time he sees Frisk he will feel happier and safer. If Frisk is dead Sans will realize that Frisk is dead, and he would be in a deep depression until he found the person who killed them.
Some character changes
Even though Negatale is like Undertale, everything there is blue, as it represents sadness and depression. They wear anything that is blue. Some characters don't wear Blue (but not many). The characters tried to be happy, doing a fake smile while their eyes are still sad, but it doesn't work. When Frisk came, they (Frisk) decided to make the Monsters have positive emotions and make them feel better.
The Dreemurr family are like Shadow figures, but they are Blue as well. The Dreemurr family keeps the positive energy from themselves.
The Skelebros (Sans and Papyrus) are very depressed. They have tried to cheer themselves up but it didn't work.