The Triacus marauders were space pirates who preyed upon the worlds of Epsilon Indi for centuries. Their motto was: "We take what is ours wherever we go." After enduring centuries of depredation, however, these worlds united and overturned their oppressors, exterminating the Marauder race utterly – as it seemed.
Their home planet Triacus was in or near the Epsilon Indi system. The sole surviving Marauder specimen, named Gorgan, lingered here as a noncorporeal being within a cave near the surface. The cave was discovered in 2268 by the innocent Professor Wilkins of the Starnes Exploration Party. Wilkins' excavation freed this last Marauder and spelled doom for the party.
The phantasmal Gorgan exerted mind control, perhaps through the power of illusion. He caused expedition leader Starnes to contact the Federation to request transport, but neglected to provide him with a rationale and the ruse failed. The adults were too strong of character to manipulate completely.
But Gorgan also thoroughly cowed his Human victims, somehow able to sharply evoke "the fear in each one of them". As he tells the party's children, whom he purposed to lead, "Anxiety is the beast which will consume them". After he extracted from Starnes a distress call to the USS Enterprise, Gorgan drove the party's adults to commit suicide.