Queen Morgana.
The Fall of Camelot was an event when Uther Pendragon's reign in Camelot fell to his daughter Morgana.
Prelude
When Arthur, Merlin and Gwaine got the Cup of Life, they however got caught by Cenred's gaurds who then took the Cup of Life to Morgause and Cenred. 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.
The Invasion
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. Elyan and Gaius managed to escape the attack and they along with Gwaine hid in the outskirts of Camelot. Uther was then dragged into the throne room where Morgause removed his crown. Morgana then appeared and revealed her true loyalties to Uther and everyone in Camelot while Arthur and Merlin secretly watched. This 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 1)
Aftermath
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 recieved, devastating Uther even more. Meanwhile Arthur was in despondant shock and wondered why Morgana would betray them in such a manner, but Merlin encouraged him to defend Camelot. Morgana's victory didn't last very long because she was later overthrown by Arthur, his knights and Merlin (The Coming of Arthur: Part 2).