Mordred was the youngest son of
Arthur's sister Morgause.
One of the Orkney brothers, Mordred was the brother of
Gawain, Agravain, Gaheris and Gareth all
of whom perished before the
Battle of Camlann.
In some early societies, like the Picts and pre-Christian Celts, descent
was frequently traced thru the women
( paternity can be questioned, maternity less so ). Mordred,
as Arthur's sister's son, was his heir; regardless of his
paternity.
Whether or not Arthur was his father, Mordred was the heir at the time of that
'Last Weird Battle in the West.'
Unlike his brother
Gawain Mordred took no part in
the Grail Quest. Mordred and Agravain brought
Guinevere's adultery
with Lancelot in to the open, starting the chain of events which eventually
brings down the realm of Arthur.
Arthur leaves Mordred in charge while he pursues
Lancelot and in varying versions Mordred usurps the crown, kidnaps an unwilling
Guinevere, marries a willing
Guinevere, or some other permutation. Whatever, the
end result was the Battle of Camlann between
Arthur and Mordred where Mordred
died and Arthur
was mortally wounded.