Damon Morrigen

Ser Damon Morrigen is an unseen character in Game of Thrones. He is not expected to appear in the show, being long dead by the time it begins.

Biography
Ser Damon was a knight of House Morrigen and a devotee of the Faith Militant who commanded the Faith's uprising against House Targaryen's rule of Westeros. He challenged King Maegor Targaryen to a Trial by Seven in order to prove the illegitimacy of his rule, but was killed by Maegor, along with all others who participated in the trial.

In the books
In the A Song of Ice and Fire novels, Ser Damon Morrien is the commander of the Warrior's Sons - one of two military orders that comprises the Faith Militant. In the show the Faith Militant is condensed into a single order.