Warlock is a Grimoire card from the original Destiny (Destiny 1), found under Guardian → Classes. 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
Warrior-scholars of the Light, Warlocks devote themselves to understanding the Traveler and its power. A Warlock's mind is an arsenal of deadly secrets, balanced between godhood and madness. On the battlefield, those secrets can shatter reality itself.
Grimoire text written by Bungie, shown as it appears in the Destiny 1 game files.