From Destinypedia, the Destiny wiki
Psion Operants are groups of powerful Psion, that (originally) first belongs to the Cabal Legions, in the first game. That tasked as Elite Psion units. They're mostly seen as majors in harder variations of missions, in the Scablands in a "eliminate the target" Public Event, Patrolling the Dreadnaught and uniquely designed variations may be encountered in the Dust Palace Strike.
Psion Operants function similarly to normal Psions except with higher durability, more frequent use of their Psionic Blast ability, and a higher chance of wielding high caliber Cabal weaponry such as Projection Rifles. Psion Operants also tend to be more aggressive, rushing down the opponent to melee them, or get a good line of sight to blast them with their Psionic Blast.
List of appearances