It has long been suspected that Merlyn and Merlin of Camelot was the same person. Captain Britain & MI-13 #3 makes it clear that Merlin indeed is one of Merlyns countless identities.
"I who am the beauty of the Green Earth and the white Moon against the stars and the mystery of the waters, and the desire of the heart of man, I call unto thy soul to arise and come unto me... For I am the soul of nature who giveth life to the universe; from me all things proceed... And unto me all things must return".
Merlyn came back to life Brian Braddock's life to be judged in the most ancient Circle of Power.
Major Story Arcs
When Mephisto goes to The Infinite Embassy to see what thee other Gods plan on doing about The Serpent he asked Merlyn but got no response it was at this point he noticed the absence of Red King.
Merlyn has many magical powers and has also trained himself so that he as developed other powers. He is possibly omniversal, though he can make duplicates of himself.
He is one of the greatest sorcerers in Earth's history.