Ian Andrew Troi was the child of an alien life form that impregnated Deanna Troi to learn about human life, growing up within days.
Ian Andrew Troi was born to Deanna Troi in The Child after an alien life form entered her body and caused a pregnancy that developed in a matter of days. She chose to keep the child and named him Ian Andrew after her late father. He grew at an extraordinary rate, appearing as a young boy within a day.
Ian's presence turned out to be causing samples of a dangerous plasma plague carried aboard the Enterprise to grow. When he understood that he was endangering the ship, he chose to die, returning to his original form as a being of light. He told his mother he had come to learn about life.