From Destinypedia, the Destiny wiki
Primevals are a class of Taken that are featured in Gambit. According to the Drifter, they are among the oldest and most powerful Taken, hence their name. They can be summoned through the use of Motes of Darkness (75 motes in normal Gambit, 100 in Gambit Prime).
Primevals are exceptionally powerful and ancient Taken, summoned and controlled by the Drifter using Motes of Dark. These beings are not like other Taken, They are not individuals transformed from the physical reality to the Deep, these creatures are born of that shadow realm, ever hungry for the Light and angry too, their elemental composition differs entirely, such as low Sterile Neutrino Count (SNC), High-Ambient Lanbda-CDM (ΛCDM). The Drifter has been pilfering the Ascendant realm of these Primevals with his banks for "potential". Now he can create "Primeval-like" entities using both energies from the Haul and sheer force of will, those even take on the visage of the Echoes of Oryx because the Drifter thought that would be the most frightening.
At one point, The Guardian managed to gain into their possession the Heart of an Ascendant Primeval, it was delivered to the Drifter who would use it for later purposes. Primevals are fast and powerful enough to tear apart teams of Guardians, and are able to heal themselves through killing opponents. However, one weakness is that their methods of engagement are like rituals: taking out their Primeval Envoy attendants weakens them.
When the Drifter summoned a group of Primevals, their arrival was preluded by the sky turning a shade of fiery green, and seemingly splitting in half. Because of their age, Primevals are also extremely powerful, as they towered over a group of Cabal who attempted to confront the Rogue Lightbearer when he traveled along the Coastline; shrugging off being attacked by the Cabal, and bombarded by the Cabal warship flying overhead. Despite the Cabal’s ferocious assault, the Primevals came out unscathed. 
Primevals are summoned once the mote bank is filled. In Gambit. they play like any other Taken Ultra of their kind, but with much more health than many other Ultra Taken. In order to kill a Primeval, players must kill Taken Wizards Envoys to begin weakening the Primeval and kill it. Killing Envoy Wizards is the only way to begin gathering stacks of the ""Primevil Slayer" debuff OR begin a damage phase (exclusive to Gambit Prime).
In Gambit Prime, the Primeval has an invulnerability shield and in order to damage the Primeval, you must kill three Envoys scattered around the map to activate a well of Light and damage the Primeval.
List of appearances