The giant statues were built as vessels to the HotBG, binding it to the Vex and the Vex to it. I believe the Vex were creations of the Traveler in order to help with her terraforming. [spoiler]Look at the progression from Precursors, normal vex, and Descendants. Goes from very bright and lightly colored to dark and sinister[/spoiler] You really found your favorite thread, didn't you?