What is a Riven mod?
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.
Once unveiled, the Riven will become bound to a select weapon of it's type randomly, and be given between two and three random positive stats, and one potential random negative "Curse". Depending on the weapon it binds to, the stats can range for +300% base damage to less than 100%, and so on for every stat. A weapon’s "Disposition" is listed in the arsonal while you have a riven that can be equipped to it.
One final note, you have a limit to how many Riven mods you can own at one time. It starts at fifteen, and can be raised by buying riven slots, up to a max of ninety. Further Riven questions can go in #riven-hub.