Princess Rhaelle Targaryen was the youngest daughter of Aegon V Targaryen and Betha Blackwood. She had four siblings; Daeron, Shaera, Jaehaerys, and Duncan. Following Lyonel's Rebellion, she married Ormund Baratheon.
Prince Duncan Targaryen was betrothed to the daughter of Lyonel Baratheon. However, Duncan broke this betrothal when he married the lowborn Jenny of Oldstones. Lord Lyonel was furious, and declared himself Storm King, and the Stormlands seceded from the Seven Kingdoms. A rebellion against the Iron Throne followed, which ended when King Lyonel was defeated by Ser Duncan the Tall in a trial by combat. King Aegon then gave Lyonel what he wanted to prevent another rebellion, by betrothing Princess Rhaelle to Lyonel's son and heir, Ormund Baratheon.
Princess Rhaelle was the main long-term reason for Robert Baratheon, her grandson, becoming the new King of the Andals, the Rhoynar, and the First Men in the War of the Usurper. While the Baratheons already had Targaryen blood through their founder, Orys One-Hand, who was the bastard son of Lord Aerion Targaryen, it had almost completely faded away.