Crystalline entity
The crystalline entity is a space-faring predator that feeds on vast quantities of organic matter. Only one was known to exist, and it was destroyed in 2368[1].
Attributes
The crystalline entity was much larger than a Galaxy-class Federation starship. Despite its size, however, it was not more powerful than such a starship; the crew of the Enterprise were confident that they could kill it with phasers and photon torpedoes, and the ship's shields proved capable of resisting the entity's attacks[2] (although they were never tested against a prolonged assault). The shields of the freighter Kallisko, however, proved to be inadequate against a determined attack by the entity[3].
The entity was capable of interstellar travel at speeds of warp 8 or more[4]. It attacked with rays of an unknown nature that reduced organic matter to lifeless ash[5], somehow extracting the energy for the entity's use. It was capable of stripping a planet of all organic material over the course of several hours. Data speculated that mineral ores containing refractory metals might provide protection, and a few colonists found refuge in a cave lined with such minerals during one of the entity's attacks, but Dr. Kila Marr -- who studied the entity extensively -- suspected that it actually spared the colonists to avoid harming Data, mistaking him for Lore, who had collaborated with it in its attack on Omicron Theta. Lore also claimed that the entity craved the life energies of sapient beings for some reason, but his claims are dubious.
The entity communicated normally with graviton pulses[6], although it could apparently understand subspace transmissions as well[7]. The Enterprise attempted to communicate with it, but Dr. Marr interfered with the attempt by using a continuous graviton pulse to create a resonant vibration within the entity that shattered it. Her killing of the creature was treated as an unjustified act of vengeance (her son was killed in the entity's attack on Omicron Theta), effectively killing her career.
In the Star Trek Expanded Universe
Another specimen is found in Star Trek Online. It is a powerful opponent, requiring the coordinated efforts of a large player fleet to destroy.