>>715461365
>>715461584
That would be the armour it's wearing. It's unchanged in the Light and Dark Worlds, so perhaps a Darkner wearing it could survive in the Light World? Like a diving suit, but in reverse.

As for what it's made of...well, the only other object that's unchanged between worlds are Gaster's eggs. And oh would you look at that, there's a pitch-black shapeshifter that hatched from one of them that has the same laugh as the Knight...