The Hidden Swarm is a Grimoire card from the original Destiny (Destiny 1), found under Enemies → Hive. 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
Boiling from the wounded surface of the Moon, the Hidden Swarm is the Hive's outermost line of defense - a numberless legion that repels intruders before they breach the temples and shrines of the vast Hive fortress.
Grimoire text written by Bungie, shown as it appears in the Destiny 1 game files.