Though she is engaged to wed Robert Baratheon, young Lyanna Stark falls head over heels for the eldest Targaryen prince, Rhaegar. Despite the fact that Rhaegar is married and a father of two, the pair runs away together in secret, leading the Starks and Baratheons to believe that Lyanna had been kidnapped by the Mad King's son.
While Rhaegar and Lyanna annul his marriage to Elia Martell and marry in Dorne, the entirety of Westeros dissolves into chaos: Lyanna's father and brother are killed by Rhaegar's father, Rhaegar's ex-wife and children are murdered, and his father's rule is eventually overthrown. Rhaegar is eventually honor-bound to fight in the war his actions caused, dying by Robert's hand at the Battle of the Trident.
By the time Lyanna gives birth to their child at the Tower of Joy, the damage to Westeros resulting from their union is so irreparable that Jon is raised as a bastard to protect him from his Targaryen name.
