I believe he became a wizard who loved living with the world of nature. He had power in his hands and voice to harness the wind, rain, sun and storms. He could conjure spells and illusions which could fool the wisest. The energy from his palms had the ability to cure disease of the human and animal body.
Legends states there came a time Merlin, in his heart could no longer carry the cruelties of war: the actions humans did to one another. He left the kingdom to go into the forest to live among the beasts. Sometimes he would be seen wearing a cloak of feathers.
When his sister Gwendydd, who ruled Cambra with her husband King Ryddrach, heard of her brother's departure, she sent message promising him great riches if he would return to her court. Merlin denied her request.
Again Gwendydd sent a message, this time telling Merlin that since he abandoned his wife she was living with great heartache. Merlin returned, only wanting to visit and explain his reasoning to his bride.
His sister had him chained to a post. While chained Merlin broke into a fit of laughter watching King Ryddrach fawning over his wife. "My sister is untrue to you, she spends the mid-sun with her lover among the groves," he said to the king.
Gwendydd knew what her brother said was a fact and released the wizard, declaring him mad.
Merlin living in the deep woods once again, received word from his sister that his wife would like to wed another. Merlin gave permission as long as he never had to see her new husband.
Merlin rode into the kingdom on a mighty stag, on his ex-wife's wedding day. He saw the bride and called out to her. Behind him, he heard the groom snicker. Furious for his lack of respect, Merlin took the groom's life force by breaking an antler from the stag and driving it deep within.
Gwendyyd, fearing her brother's safety, commanded her men to build a fortress inside the largest oak tree they could find. She wanted seventy windows and seventy doors constructed, so he could spend his days mapping the stars and observing nature's laws.
When his sister returned to her kingdom, her husband had died. After mourning her mate, she joined her brother in the oak tree and spent her days observing the patterns of the heavens, learning all of Merlin's magical gifts. She soon outgrew the teacher. Being a seer, Merlin already knew that his sister would carry on the prophetic wisdom.