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. Unlike other Taken, they are not transformed from physical reality to the Deep; they are born of Darkness, ever hungry for the Light and noticably angry. Their elemental composition differs from regular Taken, with a low Sterile Neutrino Count (SNC) and high ambient Lambda-CDM (ΛCDM).
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. They towered over a group of Cabal who attempted to confront the Rogue Lightbearer when he traveled along the Coastline, emerging unscathed from the ensuing Cabal attack and bombardment by the Cabal warship flying overhead. 
Rather than stealing the Ascendant realm of Primevals, the Drifter uses his banks for "potential" and can create "Primeval-like" entities using both energies from the Haul and sheer force of will. These can even take on the visage of the Echoes of Oryx because the Drifter is theorised to believe Oryx to be most frightening Hive. When The Guardian acquired the Heart of an Ascendant Primeval, it was delivered to the Drifter who would use it for later, unknown purposes.
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. In order to damage the Primeval, players must kill three Envoys scattered around the map to activate a well of Light and damage the Primeval.
List of appearances