It's interesting that you mention the triangular shapes that make the ghosts, since there's also a sphere in the center. And when the ghost uses its powers, the sphere emits light that pushes the pyramids outwards. This light is similar to the light that the traveler sends at the end of the Destiny 2 campaign. One possibility is that the ghost is some combination of light and dark. Somehow the two powers came together to make a hybrid. Before I get spammed with "you're wrong" messages, I want to say that this is just fun speculation. I'm not trying to prove anything about the lore.