Ian Andrew Troi II was an alien entity that appears in Star Trek: TNG episode, "The Child". Originally appearing on the Enterprise-D as a speck of white light, the entity entered the body of Counsellor Deanna Troi, taking on a physical existence as a foetus within her womb. The foetus developed fully in over a day and was born without any discomfort or difficulty to itself or Deanna. Deanna named the child "Ian" after her human father.
At first, Troi did not realize that Ian was an alien. Jean-Luc Picard was initially worried that the child could present a danger, but he appeared to be completely harmless. He grew from an infant to an eight-year old in one day. At the same time, a plasma plague put everyone on the ship in danger. A contaminated specimen of the hazardous strain had begun to grow in the cargo bay, due to radiation from an unknown source.
Ian turned out to be the source of the radiation. When the young boy realized this, he decided to sacrifice his life to save the crew. As Troi looked on, Ian died, but not before he told her that he was the alien and that he wanted to experience the Human life cycle, including death. His body reverted to its original state and left the ship, but not before he had experienced Humanity, and Troi had experienced the joy and sadness of parenthood.