The Sept Of Baelor's explosion was a turning point for many characters in Game of Thrones, especially Cersei Lannister, who established herself as the greatest antagonist of the show since so many people died in it. The Great Sept was an ancient building in King's Landing and the house of the Faith of the Seven, a religion that became a key part of the series. The Sept Of Baelor was one of the most sacred places in Westeros and where the most ambitious ceremonies of King's Landing took place, including Joffrey's wedding and Tywin Lannister's funeral.
After Cersei suffered at the hands of the High Sparrow and was forced to do the Walk of Shame, it was just a matter of time until she turned against the Faith, waiting for the right moment to strike. To evade her trial and get rid of the enemies that stood in her way to the Iron Throne, Cersei orchestrated the explosion of the Sept Of Baelor with the use of wildfire, which not only killed her biggest opponents in a matter of seconds but also annihilated all the surrounding buildings and innocent citizens who stood nearby, causing irreparable damage to the Lannisters' reputation and King's Landing.
Queen Margaery Tyrell
Queen Margaery's death was expected due to her dangerous strains with Cersei, even though she came close to getting the upper hand over her opponent. After her marriage with King Tommen was consummated, Margaery and Cersei found themselves in a battle over the king's attention, and since Tommen was still young, he easily fell for Margaery's seductive manipulation, gradually turning against his mother.
Cersei knew the only way to win over Tommen was to keep Margaery away from him, so she played the strings to get the Queen arrested under the High Sparrow's accusations of false testimony. Margaery proved to be one of the smartest characters in the show by using the High Sparrow's influence in her favor, uniting the crown and the faith and, with that, causing the High Sparrow to call off her atonement. However, Loras still had to go under trial alongside Cersei, and Margaery realized too late the reason why Cersei wouldn't show up.
Lord Mace Tyrell
Lord Mace Tyrell orchestrated one of the smartest moves of the show by having his daughter Margaery marry Joffrey and, eventually, Tommen, ensuring House Tyrell a favorable position on the side he believed was the strongest in the war. He was right about the Lannisters' power but failed to see how unpredictable the family was: Joffrey died, Tyrion killed his father, and Cersei turned her son's council into ashes. Lord Tyrell was assigned as Master of Coin by King Tommen and attended the trial at the Great Sept not only to witness his son's sentence but to try to talk his son out of the Faith Militant.
Sir Kevan Lannister
Sir Kevan Lannister was Cersei's uncle and the final obstacle on her way to the Iron Throne: she knew that even if she succeeded in getting rid of her worst enemies — Margaery, High Sparrow, Lord Tyrell — Sir Kevan would never forgive her, and as the Hand of the King, he had the power to get her either arrested or killed. For this reason, Cersei didn't lift a finger to warn her uncle about the Sept's explosion, even though the Lannister family was already cut short enough. Viewers get a final glimpse at Sir Kevan when he tried to lead Lord Tyrell toward the door.
Loras Tyrell
Apart from Cersei's personal feud with Margaery and the Faith, Loras Tyrell was perhaps the reason why the explosion at the Sept of Baelor happened, since his presence in the trial not only summoned all of Cersei's enemies in the same place but also the people who could exact revenge of her later, such as Sir Kevan and his father, Lord Tyrell. Loras was an exceptional warrior and played a big role in Blackwater's relief force, but his affair with Renly Baratheon brought the Faith's unwanted attention, which caused his arrest in season 5.
Loras' love for Renly remained until his death, evident by the emotions that assailed him when he mentioned Renly's name at the trial. Minutes before the Great Sept was blown into pieces, Loras laid out his sins and renounced his titles, joining the Sparrows and becoming Brother Loras in the final moments of his life. Loras' confession also made Margaery the heir to Highgarden, a title that also quickly went to waste once the wildfire was triggered.
High Sparrow
Of all the causalities of the Sept of Baelor's explosion, the death of the High Sparrow certainly was the most satisfying to Cersei. The High Sparrow was one of the most contentious figures in King's Landing: while he was just a low-key character in the show's overall story, he forced Cersei to go through hell, acting as her archenemy during seasons 5 and 6. The High Sparrow tricked Cersei with his false humility and tenderly manipulated her into reestablishing the Faith Militant: while Cersei thought she was turning him into her pawn, he was playing her the whole time.
The High Sparrow ordered Cersei's arrest after she was double-crossed by Lancel, her cousin and former lover, who confessed their impure activities together. By that time, Cersei gave the High Sparrow too much power and had no option but to comply with his demands, which included a naked march of shame in the streets of Kind's Landing. The High Sparrow underestimated Cersei on the day of her trial: he was positive that even with her absence, Cersei would be condemned by the gods. The next thing he knew, he was engulfed in the wildfire flames.
Lancel Lannister
In season 5, Lancel Lannister joined the Faith Militant and got a seven-pointed star carved into his forehead as a sign of his initiation. His sins became Cersei's sins, which caused her arrest and public humiliation, igniting violent confrontations between Cersei's Mountain and the guards of the Faith. On the day of the Sept of Baelor's explosion, Lancel was assigned to fetch Cersei but got distracted by Qyburn's little birds: the children ambushed him and stabbed him in his spine. Lancel died crawling toward the source of the wildfire, failing to get there in time.
Brother Boake
Brother Boake was a minor character introduced in season 5. He joined the Faith Militant and became one of High Sparrow's most loyal servants, taking part in the raid of Littlefinger's brothel and escorting figures of high importance in and out of the Sept of Baelor, including Cersei and Margaery. Brother Boake was present in Loras and Cersei's trial, closely guarding the defendant.
Citizens In And Around The Great Sept Of Baelor
The explosion of the Great Sept of Baelor killed around 200 people in a matter of seconds, including the majority of King's Landing nobility and all members of the Faith Militant (via The Washington Post). All the surrounding buildings were incinerated by the wildfire, as well as dozens of unsuspecting citizens who lived or wandered nearby. Those who weren't consumed by the green flames were hit by the debris, including an innocent who got hit by one of the Sept's giant bells.
The Aftermath of The Great Sept Of Baelor's Explosion
The explosion turned out to be both a curse and a gift to Cersei Lannister: while she succeeded in eliminating her foes, she lost another son, hinting at Maggy's prophecy drawing to a close. After witnessing his reign being reduced to ashes, King Tommen took his own life by jumping off his bedroom window. Cersei ordered his ashes to be buried at the Sept's ruins, next to her other deceased family members. Cersei claimed her right of conquest and was crowned Queen of the Seven Kingdoms: the remaining servants and citizens of King's Landing feared her so much that a coup was out of the question.