The key parts of The Crystal Cave by Mary Stewart.
Merlin is the grandson of the king, although he often goes unacknowledged by everyone except his friend Cerdic, a servant. He discovers that he can see things before they happen and wants to discover who his father is, but his mother won't tell anyone.
I will return every day.
Merlin is out exploing the woods one day and discovers the crystal cave where he meets the old and wise Galapas, who teaches him many things, including more about Merlin's magic sight, which comes from the god Myrridan.
Knowledge is very important, Merlin. I will teach you everything you need to know.
One day, Merlin returns home from the crystal cave only to find that his friend Cerdic and the King have both died. Merlin realizes his uncle Camlach will have him killed if he stays, so Merlin leaves and finds himself traveling to the kingdom ruled by Ambrosius.
Merlin discovers that Ambrosius is his father and tells him of all the visions he has, even though he has no control of what he sees or when.
Yes Merlin Emrys, I am your father.
You have the sight, Merlin. Can you see anything concerning the fate of Greater Britain?
Please father, how may I be of use to you?
I can only if the god Myrridan allows it. I do not know when a vision will come to me.
No. I can tell you why your walls are cracking. Underneath your fortress there is a cave. Inside of it there is a pond; and lurking inside is a beast. I can tell you the fate of your kingdom.
King Vortigern wanted to sacrifice Merlin to prevent their fortress walls from cracking, but Merlin convinced him to let him prophesy the future for him. Merlin predicted the victory of the Dragon (Ambrosius' army).
Ha.You will be sacrificed and you blood shall protect our fortifications.
After the death of Ambrosius, Merlin's uncle Uther is crowned King. He doesn't always agree with Merlin, but Merlin served Uther as he did Ambrosius. Merlin assists Uther in courting the lady Ygraine and predicts the birth of Arthur, who will be the once and future king.