The dikironium cloud creature was an intelligent, predatory, non-corporeal lifeform that fed by forcibly extracting the corpuscles of iron-based blood until the prey died. It received this name from the most common element it could transform itself into, the element dikironium, a substance previously only produced in the laboratory and never encountered in the field.
The cloud creature, according to scans from the USS Enterprise, existed in a borderline state between matter and energy, with elements of both. Science officer Spock theorized that it utilized gravity fields for propulsion, especially for traveling through space at faster-than-light speeds. The creature had other numerous abilities which seemed incompatible with life as previously encountered. Spock's scans revealed the creature could "throw itself out of time sync", enabling it to be "elsewhere" during phaser or photon torpedo strikes that made such weapons completely ineffective. This "time-sync" ability, perhaps moving in and out of our space-time continuum, and its utilization of gravity fields also made deflector shields useless against the creature's attacks, and may explain how the creature entered a humanoid body to completely drain it of red blood cells. It also seemed to sense when it was being scanned and could temporarily change its molecular composition into other elements to evade such searches. The vampire cloud gave off a sickly-sweet odor, best likened to honey, and some survivors of its attacks described the experience as intensely cold and almost as if they were being smothered in honey.
One of its few weaknesses was that it found humanoid blood based on copper highly unpalatable (such as that of Vulcans), and it would immediately leave such a being alone and largely unharmed after initial contact.
The creature was first encountered when it attacked the USS Farragut in 2257, taking the lives of over two hundred crewmen, including its captain. Eleven years later, one of the Farragut survivors, Captain James T. Kirk, discovered the creature on Argus X, and after it killed several of his crewmen he devoted considerable time to hunt it at the expense of an imperative medical delivery he was supposed to perform to another colony. When his first officer, Spock, and his chief medical officer, Dr. Leonard McCoy, confronted Kirk about this course of action, Kirk argued that the creature, which he was convinced was the same one that had attacked the Farragut, posed a grave threat to life in the United Federation of Planets.
Kirk claimed he could sense an intelligence, and even "an evil", about the creature when it had attacked him and his shipmates aboard the Farragut. Spock and McCoy remained unconvinced of this claim (the former wondering how Kirk could sense what he did, the latter wondering if Kirk's loss of consciousness had led him to merely believe he had). However, neither could deny the creature's fearsome abilities or its habits and therefore its semi-sentient attributes, making the creature altogether undeniably dangerous.
When the creature was spotted, the USS Enterprise pursued and attacked it to no avail, while it retaliated by attacking the ship and entering it. Chief Engineer Scott attempted to affect it by flushing some of the ship's radioactive waste into the ventilation systems. The creature later fled, either from the waste's effects, its encounter with an unpalatable Vulcan, or for different motives entirely, but not before it had inflicted more casualties. Now with conclusive proof of the creature's intelligent and aggressive predatory nature, Spock concluded that it was indeed a serious menace that had to be stopped immediately. This conclusion was reinforced by findings suggesting that the creature was about to spawn, and through fission in "thousands of parts," thereby profoundly compounding the emergency.
The Star Trek Concordance refers to this creature as a "vampire cloud".
The attack on Farragut is depicted in the comic book Debt of Honor. Kirk separates Farragut's saucer with the help of a Vulcan scientist aboard, T'Cel, enabling the surviving crew to escape. Several decades later (not long after Star Trek IV: The Voyage Home), he works together with Captain Kor and T'Cel (revealed to be half-Romulan, and now a warbird CO) to destroy another cloud creature before it can cause any more damage.