The Sept of Remembrance was a sept on the Hill of Rhaenys in King's Landing.
It received its name for being constructed in memory of Aegon I's sister-wife Rhaenys Targaryen, after she died fighting in Dorne, which is why it was located atop Rhaenys's Hill.
Construction of the Sept of Remembrance began during the reign of Aegon I Targaryen. It became the base of operation for the Warrior's Sons during the Faith Militant uprising.
On the thirtieth day since the trial of seven between Ser Damon Morrigen and King Maegor I Targaryen, hundreds of Warrior's Sons gathered for their morning prayers. Maegor mounted his dragon, Balerion, and unleashed dragonfire upon the Sept of Remembrance without warning. As the sept burned, some tried to flee, but were cut down by the archers and spearmen that Maegor had ready. It is said that the screams of the burning and dying men were heard throughout King's Landing. The burning was so immense that scholars claim there was a pall over the capital for seven days.
Afterwards, Maegor had the ruins of the Sept cleared, and ordered the construction of a large domed stable, the Dragonpit, where Targaryens could house their dragons.