The Fall of Camelot was an event when Uther Pendragon's reign in Camelot fell to his daughter Morgana. It was a major turning point in the Camelot-Essetir War, as all previous battles had been won by Uther's forces.
When knights, along with Sir Leon trespassed in Cenred's kingdom, Cenred had them killed for trespassing except for Leon, who was revived by the Druids using the Cup of Life. After Leon returned to Camelot, Uther ordered Arthur to get the Cup of Life in order to prevent it falling in Cenred's hand. Arthur subsequently left the following day with Merlin to retrieve it. However, Morgana overheard Uther and told Morgause, who wanted the Cup of Life, knowing that it would help them capture Camelot. After entering Cenred's kingdom, Arthur and Merlin were captured by a slave trader named Jarl and encountered Gwaine again while they were held captive. Jarl then forced Arthur and Gwaine to fight each other or risk getting killed by him but was foiled when Merlin created a fire with his magic, allowing Arthur, Merlin and Gwaine to escape.
When Arthur, Merlin and Gwaine got the Cup of Life from the Druids, they got caught by Cenred's guards who then wounded Arthur's leg and rendered him unconscious with a poisoned arrow and took the Cup of Life to Morgause and Cenred, forcing Merlin and Gwaine and to go into hiding with the unconscious Arthur in order to avoid detection. This happened when Jarl discovered Arthur's identity and told Morgause and Cenred where they were going before getting killed by Morgause as a "reward" for giving Arthur's location. Morgause then made Cenred's army immortal using the Cup of Life and upon deciding she no longer needed Cenred, she betrayed him and had him killed by one of the immortal soldiers to gain full control over the Immortal army, despite him begging her to spare him. Once Arthur regained consciousness the following day, with Merlin having failed to heal him with magic, he made his way back to Camelot with Merlin and Gwaine despite struggling with his wound.
Uther was soon aware of this when Leon discovered that the army was immortal and he ordered the knights to defend Camelot in whatever way possible. When Uther asked Gaius how Cenred got to know of the Cup's location, he was told by Gaius that there was a traitor in their midst, still unaware that it was Morgana, who smiled gleefully behind Uther's back. As the Immortal army arrived in Camelot, Morgana then visited Gwen and told her that the army would only kill those who attempted to resist and spare those who were prepared to welcome change in order to gain Gwen's trust, unaware that Gwen had secretly turned against her, already being aware of both Morgana's magical powers and true loyalties. Morgause then used the immortal army to seize control of Camelot and many knights and citizens were killed as a result while some including Leon were captured. Due to losing his army, Uther was then defeated and captured.
Arthur, Merlin and Gwaine then returned to Camelot after discovering the knights and citizens who were killed by the immortal army. Elyan and Gaius managed to escape the attack and they along with Gwaine hid in the outskirts of Camelot. After Arthur received treatment for his wound from Gaius, Arthur and Merlin went to look for Uther, having been told by Gwaine and Elyan that he was alive. Uther was dragged into the throne room where Morgause removed his crown. Uther angrily raged that Morgause had no right for the throne but Morgana appeared and confirmed that she had as she was his daughter, revealing her true loyalties to Uther and everyone in Camelot while Arthur and Merlin secretly watched. Learning the truth about Morgana left Arthur and Uther shocked and heartbroken. After this, Morgana was crowned Queen as Arthur, Merlin and the captive Uther watched in horror and she used this position to slowly destroy Camelot (The Coming of Arthur: Part One).
Uther was then reimprisoned while Arthur and Merlin were forced to escape and hide in the outskirts of Camelot, along with Gaius, Gwaine and Elyan. After a week, Morgana began to execute many innocent people when trying to force Leon and the other knights to serve her, along with emotionally tormenting Uther by mirroring his own actions in executing innocent people just for having magic. Morgana later told Uther that he would also be executed but only after she was satisfied with the amount of suffering he received, devastating Uther even more. Meanwhile Arthur was in despondent shock over Morgana's betrayal and Uther's lie to him about her true paternity, but Merlin encouraged him to defend Camelot, reminding him that he still had a duty to his father and people. Leon later escaped from Camelot with help from Gwen and they joined Arthur, Merlin and the others, who were also joined by Lancelot and his friend Percival. Morgana's victory didn't last very long because she was soon overthrown by Arthur, his knights and Merlin as they rescued Uther and destroyed the immortal army, forcing Morgana to escape with Morgause (The Coming of Arthur: Part Two).