In The Dark Tower Series
Walter O’Dim is a man of many identities. Self-trained as a sorcerer, he has the ability to change his appearance on a whim; this is a great advantage in his dark dealings. Working as an undercover agent for the Crimson King in Gilead, he is known as Marten Broadcloak. He stays in contact with the Red King by use of a jagged piece of smoky quartz.
Broadcloak serves as the sorcerer and advisor to the Lord of Gilead, Steven Deschain. This is a task given to him by the Crimson King in an attempt to cause the destruction of the Line of Eld, the Dark Tower and, ultimately, the White. In his position, O’Dim also has the opportunity to aid John Farson in his plans to overthrow Steven Deschain as Lord of Gilead. Both tasks, he believed, would lead to the same eventual outcome.
Walter O’Dim’s overall plan was put into motion when he, Marten Broadcloak, adviser to the Lord of Gilead, began having relations with the Queen of Gilead, Gabrielle Deschain. For two years Marten and Gabrielle carried on this way.
The crescendo came when Roland learned of his mother’s sins and father’s cuckolding. The news filled him with a rage, as Marten had hoped, and Roland foolishly opted to take his test of manhood. Broadcloak believed that the fourteen-year-old Roland Deschain would fail the test and be exiled to the west; or, even better yet, killed. To Broadcloak’s dismay, Roland passed his test and became to youngest Gunslinger apprentice to win his nickel-plated guns; besting his father’s earlier record by two years.
After passing his test and receiving his guns, Roland set off to kill Marten; however, his father, Steven, stopped him. Fearing for his son’s life, Steven Deschain issued a warrant for Marten Broadcloak’s arrest, his treachery now revealed. Turning the palace guards to dogs, Broadcloak was able to escape and flee Gilead. Although his alliance with John Farson had been discovered, his true devotion to the Outer Dark and the Crimson King was still unknown. Walter O’Dim swore, one day, to kill Roland Deschain.
A powerful wizard, Walter can easily cast illusions to disguise himself or his surroundings and is able to physically transform himself into a crow. Whether his shape changing abilities extend beyond merely his crow form remains to be seen. His capacity for sooth saying borders on the prophetic, and he can predict the future to a limited degree. He has also shown himself to be a capable necromancer, wrenching souls from the great beyond and placing them back within their decaying bodies.
What makes Walter so dangerous however, is his ability to influence and goad others into doing his bidding. Almost certainly buffed by his mystical nature, Walter can convince nearly anyone of anything, and has used his silver tongue to turn entire towns against Roland and his ka-tet in their quest for the Dark Tower. In addition to his plethora of supernatural abilities, he can transport himself to other dimensions and using sorcery, traverse great distances by opening doorways he's drawn with chalk.
Walter also posses a quasi-immortality that certainly has roots in his magical nature, or possibly stems from his servitude to the Crimson King. Walter can die, but certainly not by any natural causes (old age, disease, etc.), and the thought of a mere mortal killing him is almost laughable.
In Other Literature
The Stand- One of his appearances in King's multiverse has him leading a city of outcasts in Las Vegas, Nevada. His agenda is to rise up and rule over the survivors of "Captain Tripps", the deadly virus that decimates the population of the world.
The Eyes of the Dragon- An evil wizard who causes trouble through Delain, a medieval country.