>>532808154
It's safe to assume the black shard os made of the same stuff as the weapon and the armor of the knight, and the shard is the same in both worlds. The armor could be a physical thing in both worlds and may also let the wearer still do darkworld magics in the lightworld like some sort of power suit. That would explain why Undyne didn't lay a teen out flat instantly, it's because the suit is a major upperhand in the lightworld. Plus it was night time. There could be a special thing to the suit where it may not work in bright light in the lightworld.