~ Mists of Avalon ~
Uther went to Gorlois after falling in love with Igraine.
Whilst his troops were off battling with Gorlois,
Merlin magically altered Uther to appear like Gorlois,
so that he could get past Gorlois's guards at Tintagel
and visit with Igraine.
From this union Arthur was conceived.
After Gorlois died in battle, Igraine married Uther.