Nightmare sans had a dark past due to being bullied since they were the guardian of the negative side of the tree of feelings... They don't like to say much about their past it reminds him of when he was weak. Nightmare stands at 4'11 and is a skeleton covered in black, ink-like goop with 4 tenticals out of his back which act like murder weapons or an extra limb... He considers himself as a king of negativity. He has a sadistic ego and only let's his softer side show around people he trusts.