According to a prophecy, Balor, the god of the underworld was to be killed by his grandson. He locked his daughter, Ethlinn, in a tower made of crystal to keep her from becoming pregnant. However, Cian, with the help of the druidess Birog, managed to enter the tower. She gave birth to a son, Lugh, by him, but Balor threw him into the ocean. Birog saved him and gave him to Manannan mac Lir, who became his foster father.
It is also an Iranian name. In Persian Kian means generations.