First Appearance: Wonder Woman #54 (1952)
Creators: H.G Peter and Robert Kanigher
Abilities: Possession of the powerful sword Excalibur. Capable of altering reality to his will with powerful magics. Time travel. Can teleport to other realities and dimensions. Commands an army of beasts and monsters which obey his will. Can transform anyone into anything he wishes.
Backstory: The evil wizard of Castle Sinister who plagued the knights of the Round Table for decades. Merlin was a seeker of ultimate magics who would commit any tabboo for the sake of more power. Wishing to possess Excalibur for himself, he would be a constant thorn in the side of King Arthur and his knights of the round table, attempting to destroy them so he may claim its power for his own. Going so far as to send the knights through a portal in time where he expected his evil to be law.
With a willingness to destroy anyone who got in his way and a menagerie of minions both mortal and otherwise, there were few who could possibly oppose him.
In the post crisis continuity, it would be an elderly Merlin towards the end of his years who would decide to join the dark side of the magic force. Turning and focusing his efforts towards the destruction of the Amazons and Wonder Woman, all for the bidding of his dark master Neron and compatriot in evil magics, Morgan Le Fay.
In the Rebirth continuity, Merlin having given up his soul, would decide that free will would doom reality, and it could only continue to exist under his iron fist. Thus he sought to slay all who would oppose him and conquer reality to serve as his kingdom.