Mordred, you must come with me to Lothian. You are a descendant of royalty.
That name, Mordred
How about you call me your son
A not so warm welcome
Welcome, Ask of me a boon and it is yours.
Arthur was deceived into having a child with his half-sister Morgause, because of this sin Merlin predicted that Mordred would eventually bring Arthur's downfall. It was his FATE. In an attempt to avoid his fate (as if it's avoidable) Merlin instructs Arthur to send a boat full of babies to sea.
My promised boon
Finally, I'll be at peace.
Mordred was found by his fishermother and father, washed up ashore as the lone survivor. He was named Tad and he seemed destined to live his life with his new parents. This all changed when Nyneve showed and informed them of the change of plan.
The inevitable battle
After spending many miserable days at Lothian, Mordred finds himself to his newly discovered father. He reunites with familiar faces from Lothian and he meets new influences on his life. Eventually he can't take his new life at Camelot and decides to go on a quest for the holy grail.
A father-son moment
Mordred, My son
After going on a journey that seemed to break him further than he already was, Mordred returns to Arthur with a request. He wants to give someone his soul, someone he can trust. Mordred asks Arthur reminding him of his boon. True to his word Arthur agrees and follows through.
If this is your request then I must fulfill it.
This is it, my actions have led me to this moment. I'm to blame for all this.
Tensions arose in Camelot. Factions arose amongst many men including Mordred. This led to the destined battle that Merlin foreshadowed. Arthur spears Mordred, but Mordred is able to pull himself towards Arthur and delivers a blow.
Arthur wakes up in disbelief. Fate is not all powerful after all, he survived. A raven appears at his feet and it speaks to him. It's Mordred his son. In the end, Mordred finally got his one desire, his father claimed him as his son.