Game of Thrones : Viserys's Reign

Fifteen years ago, the Seven Kingdoms were torn apart by a civil war, known alternately as "War of the Usurper." Prince Rhaegar Targaryen kidnapped Lyanna Stark, arousing the ire of her family and of her betrothed, Lord Robert Baratheon. The Mad King, Aerys II Targaryen, had Lyanna's father and eldest brother executed when they demanded her safe return. Her second brother, Eddard, joined his boyhood friend Robert Baratheon and Jon Arryn, with whom they had been fostered as children, in declaring war against the ruling Targaryen dynasty, securing the allegiances of House Tully and House Arryn through a network of dynastic marriages (Lord Eddard to Catelyn Tully and Lord Arryn to Lysa Tully).

The civil war climaxed with the Battle of the Trident, when Prince Rhaegar was killed in battle by Robert Baratheon. The Lannisters finally agreed to support King Aerys, but then brutally turned against him, sacking the capital at King's Landing. Jaime Lannister of the Kingsguard betrayed and murdered King Aerys.
Robert Baratheon was declared King of the Seven Kingdoms. Unfortunately, during the war, Lyanna Stark had died, apparently of illness shortly after her brother captured the fortress where she'd been held captive, Robert Baratheon instead married Cersei Lannister to cement the alliance with her House. Despite Robert's victory, the Mad King's younger son Viserys and only daughter Daenerys and his granddaughter Rhaenys Targaryen were taken to safety across the sea by loyal retainers. After the war House Martell chose a path of isolation, since Prince Doran's sister Elia Martell (Prince Rhaegar's wife) and her young son had been killed by knights sworn to House Lannister during the storming of the capital.

Years later treachery tears apart the fragile realm and Westeros has fallen into rebellion as multiple kings rise up. Meanwhile the Targaryens stay in Essos, gaining power and influence as Westeros lords are too busy warring.