Dikironium cloud creature
The dikironium cloud creature was an intelligent, predatory, non-corporeal lifeform that fed by forcibly extracting the red corpuscles from 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 , 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 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. When witnessed, the creature's size measured approximately ten to sixty cubic meters, fluctuating in size as it moved. Its composition resembled a gaseous cloud, partially transparent and partially dense. 

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  in 2257, taking the lives of two hundred crewmen, including Captain Garrovick.

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 hunting it at the expense of an imperative medical delivery his ship 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 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 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  in "thousands of parts," thereby profoundly compounding the emergency.

Eventually, the Enterprise crew followed it to the planet Tycho IV, where the creature had fought the Farragut, and managed to kill it with a massive antimatter explosion. ()

Appendices

Background information 
This creature was never given a formal name on screen, and was described by a multitude (over thirty) of "creature" references, including "that/a/the creature", "a strange, gaseous creature," "an intelligent creature", and "Jim and his creature". It was also referenced as a "cloud", in as far as being a "gaseous cloud", "a strange cloud", "cloud, cloud", and "a strange cloud of chemical elements". 

The whole name "dikironium cloud creature", used here, first originated from the ; whereas the Star Trek Concordance (2nd ed., p. 140) referred to it as a "vampire cloud" catalogued together with various others under "cloud".

Apocrypha 
According to the novel Orion's Hounds, a predatory cloud creature with metadimensional abilities was genetically engineered to have warp capability and a hunger for humanoid blood, so that it could be used as a biological weapon. The clouds nearly destroyed both sides in the war before they were stopped, but a few managed to escape. Another vampire cloud is said to have battled the Klingons in the 2310s and was ultimately destroyed.

External link
 

de:Dikironium-Nebelkreatur
Non-corporealsUnnamed individuals
Non-corporeal speciesUnnamed species
Memory Alpha articles named from supplementary resources