The dikironium cloud creature (also referred to as the "vampire cloud") was a semi-sentient gaseous predator that sustained itself by feeding of the corpuscules of iron-based blood until its prey died. The creature was given this name because of the most-common element that the creature was able to transform into, namely dikironium.
According to sensor scans taken by the USS Enterprise in 2268, the creature seemed to exist in a borderline state between matter and energy, and contained elements of both states. It was also theorized that the creature propelled itself at warp speed using gravity fields.
It was also revealed that the creature found copper-based blood unpalatable and would immediately leave that prey alone. This was discovered when an attack upon the Enterprise's Vulcan science officer, Spock, was aborted. (TOS episode: "Obsession").
The first Federation encounter with the creature took place in 2257, when the USS Farragut arrived in orbit of Tycho IV on a survey mission. After killing a landing party on the surface, the creature proceeded to board the Farragut and kill 200 crewmen, including Captain Garrovick. (TOS - My Brother's Keeper novel: Constitution)
The next encounter with the creature occured on Argus X in 2268, when the Enterprise was performing a survey of that planet. During this encounter the creature killed several crewmen on the surface and aboard the Enterprise. Fortunately, serving aboard the Enterprise at that time was Captain James T. Kirk, who had served aboard the Farragut during the first encounter, and Ensign Stephen Garrovick, the son of Captain Garrovick.
Together the two officers worked on a way to destroy the creature, against the advice of Spock and Dr. McCoy. After pursuing the creature back to Tycho IV, Kirk and Garrovick set off a large antimatter bomb which destroyed the creature. (TOS episode: "Obsession").
In 2381, the USS Titan Chief of Security Ranul Keru devised a security drill against a dikironium cloud creature, something that first officer Christine Vale jokingly called "just mean." (ST - Destiny novel: Gods of Night)