Harington, who plays Jon Snow on the cable network's biggest show, will star in Gunpowder, a three-part historical miniseries coming to America in December (it's currently airing on BBC One in England).
Harington plays Robert Catesby, who was the leader of the Gunpowder Plot in 1605 London, which was when a group of persecuted English Catholics attempted to assassinate King James I by blowing up the Palace of Westminster. Harington, who's also a co-executive producer, has a personal connection to the role; he's a descendant of Catesby's.
This is Harington's second HBO limited series. The first was the tennis mockumentary 7 Days in Hell.
Gundpowder will air over three consecutive nights starting Monday, Dec. 18 at 10/9c on HBO.