Weird crystal-ish looking creature that lives in space and feeds on people.
The Entity was lured to the Omicron Theta colony by the android Lore. Once there, it wiped out the entire colony's population, except for the androids Lore and Data, and their creator, Dr. Noonien Soong. Soong disabled both androids and wiped Data's memory clean, replacing it with the memories of the lost colonists, before he escaped to Terlina III with his wife.
The Entity was also responsible for two deaths on Melona IV. Captain Picard of the Enterprise attempted to establish communications with it, hoping to understand its motivations. However, Dr. Kila Marr, whose son was one of the colonists killed by the Entity at Omicron Theta, destroyed the creature, getting her revenge and sacrificing her career.
See also: The Star Trek Project