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, 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 Sir Leon, who was revived by the Druids using the Cup of Life. After Sir Leon returned to Camelot, Uther ordered Arthur to get the Cup of Life along with Merlin and they were later joined by Gwaine. However Morgana overheard Uther and told Morgause, who wanted the Cup of Life, knowing that it would help them capture Camelot. When Arthur, Merlin and Gwaine got the Cup of Life, they got caught by Cenred's guards who then took the Cup of Life to Morgause and Cenred while Arthur, Merlin and Gwaine escaped. This happened when Arthur, Merlin and Gwaine met a slave trader named Jarl who told Morgause and Cenred where they were going and was apparently killed by Morgause. Morgause then made Cenred's army immortal using the Cup of Life and she betrayed Cenred and had him killed to gain full control over the immortal army.
Uther was soon aware of this when Sir Leon discovered that the army was immortal and he ordered the knights to defend Camelot. 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, unaware that it was Morgana. Morgause then used the immortal army to seize control of Camelot and many knights and citizens were killed as a result while some including Sir 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. 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 and she used this position to slowly destroy Camelot. (The Coming of Arthur: Part One)
Uther was then imprisoned while Arthur and Merlin were then forced to escape and hide in the outskirts of Camelot, along with Gaius, Gwaine and Elyan. After a week, Morgana began to execute many people when trying to persuade Sir Leon and the other knights to join her. 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 and wondered why Morgana would betray them in such a manner, but Merlin encouraged him to defend Camelot. Sir Leon later escaped from Camelot with help from Gwen and they joined Arthur, Merlin and the others. 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).