Forgive me

, but Donalbain I believe is the younger son of the murdered Duncan, King of Scotland. The elder son, Malcolm, has been named heir by King Duncan, whom Macbeth murders in order to be king. Malcolm and Donalbain flee, accused by the bloody Macbeth of their father's murder. Malcolm returns in the end to become king, who then calls home the fled friends of the throne, presumably including Donalbain. (Love those clicknotes.)
You have chosen what seems to me to be Shakespeare's most perfectly constructed play with my favorite scenes as a child. I knew the witches chant by heart.

"Bubble, bubble, toil and trouble, fire burn and caldron bubble."