The Astartes story all makes sense now! The masks, the powers, the rebellion, someone has been poking around obscure Xenos artifacts! Here is the promised full video on the Yu'Vath https://youtu.be/QXradVYY4gk

Before watching this video GO WATCH THE ORIGINAL FIRST!
https://www.youtube.com/watch?v=eoCcpMW8fSs