The Vex is a Grimoire card from the original Destiny (Destiny 1), found under Enemies → Vex. Grimoire cards held most of the game's lore and were read on Bungie.net and in the companion app, not in the game itself.
Card text
The Vex are architects of ancient and complex structures thought to be buried within every celestial body. Linked by a network unlike any on Earth, they operate in unison, directed by a single unfathomable purpose.
Grimoire text written by Bungie, shown as it appears in the Destiny 1 game files.