Arthur's birth and accession

Uther Pendragon, Arthur's father was charmed by Lady Igraine, who was the wife of his enemy Gorlois, tricked him with Merlin's help and slept with Igraine. Thus Arthur is an illegitimate child, but he was legitimated by Uther's subsequent marriage to Igraine.
As i said, Merlin helped Uther, but for this help, he asked Pendragon to give him king's first son for education and upbringing. And he managed to keep his promise.

In a few years the war was broken by the Saxon invaders. And during this war the Saxons soon contrive his death by poisoning a spring he drinks from near Verulamium. On Uther's death, the fifteen-year-old Arthur succeeds him as King of Britain and fights a series of battles.