The Great Sept of Baelor, also called the Great Sept or just the Sept of Baelor, is a massive sept, the center of religious worship for the Faith of the Seven and the seat of the High Septon of the Faith. It is located in King's Landing, the capital of the Seven Kingdoms and is the largest single building in the city, though the Red Keep complex sprawls over a larger area. Great religious ceremonies are held there.
The sept is named for King Baelor Targaryen, a king noted for his religious piety, peace-making skills and pacifism. A statue of King Baelor is located in the square in front of the sept.
When Eddard Stark agrees to falsely confess his crimes to ensure the safety of his children, Queen Regent Cersei Lannister decides to hold the confession on the steps of the Great Sept in front of a large crowd. However, rather than allow Eddard to take the black and join the Night's Watch as agreed, King Joffrey Baratheon rashly orders that Eddard be beheaded. This order is quickly carried out by the King's Justice, Ser Ilyn Payne, over the objections of Cersei and the King's other counsellors.
In the books
In the Song of Ice and Fire novels, the Great Sept of Baelor is located atop Visenya's Hill in King's Landing. It consists of the traditional seven-sided structure, albeit on a huge scale, with seven slender towers topped by crystals rising from it. There is a grand square in front of the Sept, from where the High Septon can address the masses.
Prior to the invasion of Aegon the Conqueror, the center of the Faith in Westeros was the Starry Sept of Oldtown. When Aegon captured Oldtown after his great victory on the Field of Fire, the High Septon advised Lord Hightower to surrender the city and swear fealty. The High Septon had received a vision from the Seven advising him to bless Aegon's cause. Aegon, his family and his followers converted to the Faith, burning the idols of the old gods of Valyria, but refused to give up the ancient Valyrian practices of polygamy and incest. When Aegon died and the throne passed to his son, born of incest, the Faith launched a bloody uprising against him which was eventually crushed.
Relations between House Targaryen and the Faith were repaired when King Baelor I Targaryen ascended the throne. Septon and king, Baelor was holy and devout, and ordered the building of a huge sept in King's Landing for the glory of the Seven. The building, completed after Baelor's death, was named in his honor.
According to the tradition of the Seven and the laws of the city, blood is not allowed to be spilled on the site of the Great Sept. Joffrey's execution of Eddard on the steps breaks this law and custom, to the distress of the High Septon.
According to the TV series official pronunciation guide developed for the cast and crew, "Baelor" is pronounced "BAY-lor".