The Vezda were an extremely ancient species of malevolent, extra-dimensional lifeforms capable of taking over humanoid bodies.
History
An orb holding a Vezda
Federation scientists hypothesized that, in the distant past, a highly advanced spacefaring civilization inadvertently brought the Vezda from another dimension while phase-shifting to achieve immortality. Describing them as "parasites" or "hitchhikers", the civilization trapped thousands of Vezda inside orbs, which were secured within a layered dimensional prison on Vadia IX.
The Vezda were dreaded by the Lanthanites; Commander Pelia called them "the desire to malign, to pervert, and consume, given corporeal form." In turn, the Vezda dismissed the long-lived Lanthanites as mere children. They were greatly feared by the M'Kroon, in whose language vezda meant "evil". The Vezda had a mutual enmity with the Gorn, who had a genetic compulsion to exterminate them on sight and could recognize them even within their hosts. On the other hand, the culture of the planet Skygowan was built around worshiping the Vezda as gods.
The newly-unleashed Vezda face Marie Batel as she prepares to reimprison their kind.
In 2261, the Vezda Zeperez escaped from his orb and possessed Nurse Dana Gamble from the USS Enterprise. Zeperez attempted to take over the Enterprise, but was expelled from Gamble's body and dematerialized by transporter. It was decided to keep him confined within the transport buffer, but Zeperez managed to reconstitute Gamble's body and escape undetected. He made his way to Skygowan, intending to free his brethren from Vadia IX and take the population of the planet as their new hosts. He and the other Vezda were resealed within their prison by Captain Marie Batel, when she became the Beholder. (SNW: "Through the Lens of Time", "New Life and New Civilizations")
Biology
A Vezda emerging from its host
Although intangible and undetectable to sensors, Vezda had a distinct form featuring a rounded head with split jaws bearing numerous large fangs. They were small in size, barely larger than a Human hand. A single Vezda could generate approximately 3.66×1026 watts of power, almost as much as the output of Sol.
The entry of a Vezda into a humanoid was marked by the traumatic ejection of the bulbus oculi from the orbital sockets. Possession caused brain death in the host; the Vezda gained access to all of the host's memories and thoughts, allowing it to disguise its presence. It continued to animate the body, preventing both deterioration and regeneration. As the possession progressed, the embodied Vezda gained increased strength, the ability to penetrate force fields, and some means to obtain personal knowledge from others for psychological manipulation. The Vezda could move to another host at will, or be forced out by a phaser beam set to kill. Eventually, the Vezda host became virtually impervious to physical harm and capable of firing powerful energy blasts. (SNW: "Through the Lens of Time", "New Life and New Civilizations")