During King Arthur's reign, he had accidentally committed incest with his half-sister, whom he was unaware of at the time. The result of the affair resulted in the birth of Mordred, who was destined to kill Arthur later in life. King Arthur was forced to send all newborn babies to sea to die in order to prevent that.
For you see, you can't fight your fate.
However, fate decided to let Mordred live and he was found and raised by couple whom he referred to as fishermother and fisherfather. Mordred was unaware of his true origins until Nyneve, a sorceress came to visit and told him she had to live with his real family.
He was sent to live with his real mother Queen Morgause along with his two half-brothers, Garet and Gawain. Nyneve also gave Mordred a dog to keep him company. During this time, he became aware of the prophecy that foretold Arthur's death at the hands of Mordred. When he was old enough, he left home to become one of Arthur's knights, along with Garet and Gawain. He later embarked on a quest to find a way to fight his fate.
Eventually, his quest took him to Merlin who was previously trapped by Nyneve. After his release, Merlin killed Nyneve which also resulted in the death of Mordred's dog Gull. Merlin also showed Mordred a vision of the final battle between Mordred and Arthur.
Knowing that he can't fight his fate, he learns to accept it and thus the final battle soon occurs.
However, Arthur didn't die as Merlin predicted. Arthur soon found himself in Avalon with a guard to watch over him. The guard was Mordred in the form of a bird.