The Delgonians were a telepathic race native to the same system as the Velantians, a species they enslaved and fed on. The most prominent among them were the Overlords of Delgon.
Delgonians strongly resemble their dragon-like Velantian neighbors, with the only difference being their heads, which are simian rather than reptilian. Additionally, while normal Delgonians have wings, Delgonian Overlords do not.
Delgonian Overlords wield considerable psychic abilities. They use these to mentally enslave Velantians and lesser Delgonians, controlling their perceptions and actions and reading their minds at will. They can also psychically feed off the life force of dying organisms, preferably sentient psychic beings such as Velantians. The Delgonians feed on their victims by subjecting them to a slow and excruciating death by torture, extracting their lives as they slowly die.
The Overlords of Delgon enslaved the Velantians long ago, torturing them to death in order to feed on their life force. The Overlords would force Velantians to come to Delgon and die, and Velantian champions who attempted to visit the planet and defeat the Overlords met the same fate.
This changed with the arrival of Lensman Kimball Kinnison on Delgon. He allied with the Velantian Worsel and defeated the Overlords, breaking their hold over the planet and the system. After this, the Velantians began to hunt down and destroy Delgonian Overlords whenever they made their psychic presence known, and eventually believed they had wiped out the species.
However, the Overlords survived, and later allied with the Eich. The Overlords, provided with spacecraft and a "hyper-spatial tube", began to prey on Civilization shipping, torturing and devouring their abducted victims. One of these Overlord ships was destroyed in a battle with the Galactic Patrol. One other Overlord was on the Eich homeworld of Jarnevon when Civilization destroyed it, and its death was believed to have marked the species' extinction..
However, the Eich had transported some Overlords to other worlds in the Milky Way, particularly Lyrane II. These survivors then cooperated with the Eich in supervising Boskonia's Milky Way operations, sustaining themselves largely by feeding off the native Lyranians.
The Delgonian Overlords rule over Delgon with absolute, unchallenged authority. The remainder of the planet's population, meanwhile, are little more than mind-controlled chattel. The Overlords sometimes torture normal Delgonians for food, but prefer targeting Velantians for this purpose.