Geoffrey of Monmouth
In Geoffrey of Monmouth's Historia Regum Britanniae, Igerna enters the story as the wife of Gorlois, Duke of Cornwall. King Uther Pendragon falls in love with her and attempts to force his attentions on her at his court. She informs her husband who departs with her to Cornwall without asking leave. This sudden departure gives Uther Pendragon an excuse to make war on Gorlois. Gorlois conducts the war from the castle of Dimilioc but places his wife in safety in the castle of Tintagel.
Disguised as Gorlois by Merlin, Uther Pendragon is able to enter Tintagel to satisfy his lust. He manages to rape Igraine by deceit - she believes that she is lying with her husband and becomes pregnant with Arthur. Her husband Gorlois dies in battle that same night. This basic account has been adapted into media such as the film Excalibur.
Geoffrey does not say, and later accounts disagree, as to whether Gorlois died before or after Arthur was begotten (something that might be important in determining whether or not a child could be made legitimate by a later marriage to its true father). Uther Pendragon later marries Igraine.
According to Geoffrey, Igraine also bore a daughter Anna (referred to as Morgause in other works) to Uther Pendragon, this Anna later becoming the mother of Gawain and Mordred. Yet Geoffrey also refers to King Hoel of Brittany as Arthur's nephew and presents a prophecy that to Uther's daughter will be born a line of seven kings, something true if Hoel is Anna's son, but not true if only Gawain or Mordred are Anna's sons. There is confusion here, especially as Welsh genealogies name an Anna as Hoel's mother but one not connected to Uther Pendragon.
Read more about this topic: Igraine