House Longwaters is a very minor house of the crownlands descended, twice over, from bastardy of dragon blood. Lord Alyn Velaryon, called Oakenfist, himself born of Rhaenyra Targaryen's dragonseed legitimation during the Dance of the Dragons, loved Princess Elaena Targaryen in secret, and their natural son Ser Jon Waters became a knight of renown. Jon's own trueborn son, ashamed of no bastardy himself but proud of what his father had overcome, added "Long" to the name Waters, founding the line that bears it still.
The Longwaters have carried no great titles since, though they have not forgotten their descent from Velaryon and Targaryen alike. By the time of the War of the Five Kings, Rennifer Longwaters serves as chief undergaoler of the dungeons below the Red Keep, fond of boasting to any who will listen that he carries "a drop of dragon" in his veins. It is Rennifer who explains the black cells' workings to Ser Jaime Lannister while investigating Tyrion's escape, and who later lets Jaime into Ser Ilyn Payne's chambers unremarked.