The Crystalline Entity was a powerful, spaceborne creature characterized by a crystalline structure that resembled a large snowflake. It had warp speed travel capability, formidable size, and the ability to consume all life on a planet or starship. (TNG: "Datalore")
The Entity functioned as a giant electromagnetic collector and needed constant power, converting organic matter into energy. Once the Entity ravaged a planet, the planet's natural resources were completely stripped. No further life would be able to grow for many years. Everything, including vegetation and bacteria, was completely converted. Bitrious matter was usually deposited in the Entity's wake, as were elevated levels of hydrocarbons, lanthanides, and nitrates. The Entity was unable to penetrate rock lined with both kelbonite and fistrium. Both metals needed to be present in order to refract the Entity's absorption power. The shields of a Template:ShipClass starship were also impervious to the Entity. Dr. Kila Marr was one of the Federation's foremost experts on the Crystalline Entity, studying twelve attack sites including Forlat III. (TNG: "Silicon Avatar")
The Entity, to which thousands of deaths are attributed, attacked the planet Omicron Theta in 2336, stripping all living material from the entire colony and killing many inhabitants, including Raymond Marr, Kila Marr's son. Colony residents Drs. Noonian and Juliana Soong both fled during the attack aboard an escape pod, although Juliana Soong was severely injured. (TNG: "Datalore", "Silicon Avatar", "Inheritance") The android Lore was an ally of the Entity and responsible for luring it to Omicron Theta. He tried to "feed" the crew of the USS Enterprise-D to the Entity after he was revived in 2364. The Enterprise's shields were enough to protect it from the Entity, and Lore was eventually transported into space. The Crystalline Entity retreated to parts unknown. (TNG: "Datalore")
In 2368, the Crystalline Entity destroyed the fledgling colony on Melona IV. Two colonists, including Carmen Davila, were lost in the attack. Most of the colonists survived, thanks to a cavern lined with kelbonite and fistrium. The Enterprise rescued the survivors and brought Dr. Marr to the planet so she could further study the attack. She worked with Lieutenant Commander Data in order to develop a technique to communicate with the creature.
The Enterprise intercepted the Entity after it had killed the entire crew of a freighter, the Kallisko. They attracted the Entity by emitting graviton pulses at ten per second from the Enterprise, as lower pulse rates had no effect on the creature. The creature approached the Enterprise, replying with a series of graviton pulses that the Enterprise crew interpreted as meaningful, but which it needed time to decode. Marr increased the pulse rate, eventually trying a continuous graviton emission. The Entity stopped emitting pulses of its own as the emissions drove it into resonance. The Enterprise crew tried to stop the emissions, only to find that Marr had isolated the access code for the emission controls. Before the crew could halt the emissions, the resonance shattered the Entity. (TNG: "Silicon Avatar")
Described in the script of the episode "Datalore" as , "MOVING IN to float close to our starship, the SHIMMERING CRYSTAL COMPLEXITY is very obvious. It looks both very lovely and terribly powerful at the same time.(...) The huge crystal form, two or three times the size of the starship, SLOWLY SPINNING, pictorially dominating the starship as it floats next to it (scenes 55a & 56)." , the entity was eventually visually conceptualized by Andrew Probert in October 1988 (The Art of Star Trek, pp. 64-65), though he has stated that while modelmakers usually closely followed his designs, he considered the Crystalline Entity the only exception to this rule, as the final product differed radically form his concept drawings. (Star Trek: The Magazine Volume 2, Issue 12, p. 45)
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In its very first appearance, the Crystalline Entity was one of the first CGI sequences used in the Star Trek franchise. It was rendered by Chris Alsop at The Post Group. (Star Trek: Communicator issue 148, p. 49) The very first CGI effects was the Genesis file in Star Trek II: The Wrath of Khan.
On 11 April 2012 director Robert Meyer Burnett confirmed and released the first image of the, in high definition, remastered Crystalline Entity for the Blu-ray edition of TNG Season 1.  However, as the original CGI files of the entity were lost, the model was recreated with the use of the latest CGI techniques at CBS Digital , as was confirmed by Michael Okuda, "Yes, the original Crystalline Entity was created on a computer system that probably no longer exists. CBS Digital's Niel Wray rebuilt it from scratch, retaining the original design, but making it more beautiful than ever." 
In the non-canon novel Orion's Hounds, the USS Titan encountered a number of these creatures, along with the entities seen in "Encounter at Farpoint" and other spaceborne lifeforms. These beings were given the collective term "cosmozoans".
Another Crystalline Entity exists in solitude near the Demil System in Star Trek Online.
The Crystalline Entity makes an appearance in the 1999 PC game Star Trek: The Next Generation - Birth of the Federation, although it's called the Crystal Entity.