All Exos visit it in a dream. Some visit it peacefully, while others slaughter an army made of everyone they meet. It's description in the Ghost Fragment: Legends card sounds strikingly similar to the Vex Citadel and the lands surrounding it. When you think about how Dr. Shim and his team visited the Citadel in [i]proxy, robotic bodies[/i]... a conspiracy begins to take shape ;)