Dustbringer12

Sibling can soulcast, and spren inhabit fabrials. The iron in the tower is not part of the sibling, but it is like the sibling's body because the sibling inhabits the fabrial which is the tower.

One thought I had: the black mist and the red mist do seem to be from the same thing - corrupted investiture - but the hion lines are cyan and magenta, representing a push and a pull. Maybe the two styles of corrupted investiture have something to do with a push and a pull/two sides of the same coin?

So here's the theory, someone heavily invested designed Canticle to do a thing, and it needs people to live there so that it can do the thing. They send Zellion with the hover bike technology to Threnody so he can bring people from Threnody to Canticle, enabling Canticle to do whatever it is designed to.