Merlin the wizard was assembled by a Welsh cleric, Geoffrey of Monmouth, in his book Historia Regum Britanniae, finished around 1136.

Geoffrey drew on two older Welsh figures — the mad prophet Myrddin Wyllt, who fled into the Caledonian forest after a battle and lived on grass, and the boy-seer Ambrosius, who unlocked the mystery of King Vortigern's collapsing tower.

Geoffrey blended them into one man he named Merlinus and placed at the court of Uther Pendragon and later Arthur.

In the stories Merlin arranges Arthur's birth, sets the sword in the stone, and finally lets a fairy sorceress named Nimue trap him in a cave or an oak tree — because even the wisest wizard falls in love badly.

There is no historical Merlin.

There didn't need to be.

Once the story existed, everyone else began remembering him.