Summerhall was a lightly fortified castle that Daeron II built on the Dornish marches, roughly where Dorne, the Reach, and the Stormlands come together (SSC). It is clear that some sort of tragedy happened to leave the castle ruined (III: 486, 492), and that this event was linked to the "dream of dragons" (III: 292). Rhaegar Targaryen in particular was deeply affected by "the shadow of Summerhall", because he was "born in grief". It is noted that he would sing of the death of kings there (III: 486). Finally, the Ghost of High Heart states that she "gorged on grief at Summerhall" and then demands to hear "my Jenny’s song" (III: 492).
Since Martin has unequivocally stated that the tragedy had nothing to do with Maelys the Monstrous and the War of the Ninepenny Kings (III: 752, SSC), the common interpretation of these facts is that the ruin of Summerhall took place in 259. Triggered by an unsuccessful attempt to raise a dragon, the tragedy likely includes the death of King Aegon V and possibly others while at the same time heralding or directly contributing to Rhaegar Targaryen’s birth. The Ghost of High Heart’s grief may be connected to the song of Jenny of Oldstones and the Prince of Dragonflies (III: 520). The Prince of Dragonflies was Prince Duncan Targaryen, the eldest son and (sometimes) heir (SSR) to King Aegon (SSR). A lyric from the song (III: 920) states that Jenny dances with her "ghosts", which suggests that the "grief" at Summerhall includes Duncan’s death and Jenny’s possible madness in reaction to her loss. Finally, it is possible that Ser Duncan the Tall, King Aegon’s friend and Lord Commander of his Kingsguard, also died there, given the fact that Ser Gerold Hightower was Lord Commander by 261 (III: 752).