Nightmare Sans, shortened to just Nightmare, is from the Dreamtale AU, and leader of the Bad Sanses. 

Nightmare is, to some, the complete embodiment of evil. He’s a deceitful and erratic being of pure hate. However, he is extremely intelligent and rather knowledgeable, and even cunning. His goal, is complete and perpetual torment, he’s stuck with the job of dealing with the fools in his group. He closely works with his friend, Cross. He’s also very formal, and addresses people properly.