Shan

the first wizard

"served the Dragon King for a score of years"

then (according to Lord Randolph) "left the service of the Dragon King and aided King John in battle against him," as well as working the Border Magic, "because it was in plans to attack us that the Dragon King first revealed his true purposes and his madness"

asked the unicorns for a magic ring when he was young and proud, and said he'd curse them if they didn't give one to him; they gave him a ring and wouldn't tell him its function but presented him with the Riddle of Shan's Ring instead

according to Ted, "could call the winds and make floods and control dragons"