Riven mods are special purple mods, awarded from Sorties, special alerts triggered by DE, and select quests as compensation if the frame it would award is already owned. The incarnation of RNG itself, they have several aspects to them. First, they start off as "Veiled" and give no stats when equipped, and are tied to a weapon type: Rifle, Shotgun, Secondary, or Melee. By equipping the mod into a weapon of the corresponding type, you way attempt it's "Challenge", created randomly from two parts: A Task and a Complication. These can range from "Complete a 30+ interception mission, alone." to "Get 66 finishers in a day", and once finished the mod is unveiled at the end of the mission.