he becomes completely black with bright blue glowing eyes, and his head is aflame with bright blue fire. The back of his head is also somewhat cracked. His outlines have a dark shade of blue. Nightmare Mode Sans personality is the same as classic sans, he likes to joke around and he dosent take anything seriously in his current form but if put under major threat he takes it seriously. Sans is also very lazy and he likes to make skeleton puns or any kind of pun joke.