... are the only enemy in Destiny that has one or more instances where they DIDN'T attack you on sight. The two occasions are in Paradox and Blood in the Garden, both are coincidentally key parts of the No Time to Explain Quest line.
The first is easy to explain, they can't beat the Taken alone.
But the second...
Isn't it odd that a race of killing machines that follows a strict pattern would ALLOW a Guardian to walk freely among the Black Garden, their supposed home*? It isn't until you shoot one that they start attacking.
*then again, how else was I supposed to get this nice pic?
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Well they where the Sol Divisive. These are a sub-spcies of the vex that you see almost ALWAYS worshiping. Like in the final mission of the original Destiny story, they where worshiping their god. Their belief is that worshiping equals power. Also that idea is based off Sword Logic. This was learned from Quira Blade Transform who told these vex to woriship. I think these worshipers worship a worm gods heart.