Twilightmon is Metaldramon/Signamon's Ultimate form. Like with his other forms, he is one of the members of the Royal Family, the youngest son of Queen Queen Majestymon and King Royalmon. His twin sister is Himemon, Princessmon's ultimate form.
This form is rather different in appearance with his much stronger Mega forms, but have resemblance to his later form's offspring, DemiSignamon. He doesn't seem to have a tail, but does have black angel-like wings that he later loses when he Digivolves to Mega.