The Great Battle for Camelot was what Kilgharrah called the invasion of Camelot by Cenred.
Prelude
It was almost a year after Morgause's Invasion of Camelot in which she took Morgana after Merlin gave her the antidote to the poison he gave Morgana if she stood down. During that year she corrupted Morgana turned her from Uther to her allegiance. She sent Morgana to be rescued by Merlin and Arthur who were searching for her. They found her in a camp of bandits and defeated them in battle and took her home safely. After she rested, she talked privately with Merlin, forgiving him and claimed she understood why he tried to poison her. That night Morgana met up with Morgause and gave her a handkerchief which had the tears of Uther on them. Morgause thrown it into a cauldron along with a mandrake root, which emits a piercing scream as it is thrown in; Morgause explains that the scream emitted when the root is thrown in could only be heard by those with magic. For those without magic, it could pierce someone's soul and bring their worst fears to life. Morgana planted it under his bed and while celebrating her return, Uther went outside and experienced a hallucination of Ygraine de Bois. He was found screaming in terror and taken to rest. Morgause's plan was going as planned as Morgana successfully planted seeds of doubt in the court of Camelot with suspicions that Uther had gone mad.
Morgause went to her old friend Cenred and informed him of Uther and that Camelot was weakened. Cenred was unsure stating that even with Uther incapacitated, an assault on Camelot was not to be taken lightly. Morgause reminded him of their ally in the city; when Cenred expressed his doubt about the loyalty of traitors, Morgause assured him Morgana could be counted on until the end, convincing Cenred to take the fight to Camelot. He started raising an army with mercenaries flooding into his kingdom. Arthur and Leon caught wind of this and alerted Uther that they feared he was planning an invasion. After this he saw a dripping wet Ghost of one of the many child sorcerers he had drowned in the Great Purge. He shouted at him to go away while the court saw him shouting at nothing, Uther was taken away. Merlin soon discovered that Morgana was twisting Uther and followed her to her meeting with Morgause hiding behind a rock. She told Morgause that Merlin was suspicious and revealed she knew he was hiding. He tried to escape but was captured by Morgause's henchmen. Morgause had Merlin chained up with enchanted unbreakable chains and left him to die at the hands of Serkets. Merlin initially tried to break the chains with magic, but when that failed, he resorted to blasting the Serkets back with magic, though not before one managed to wound him with its tail sting. As the Serket's venom began to affect him, he used his power as a Dragonlord and called out to Kilgharrah, using the ancient dragon language. As the Serkets moved in for the kill, the dragon drove the Serkets back with his fire breath and then carried Merlin to safety.
Invasion
Cenred began his invasion of Camelot and entered the kingdom with his 20,000 strong army, Morgause
watched events unfold with satisfaction. Gaius desperately urged Arthur to take control, but the prince refused to give up hope that Uther would recover. Kilgharrah helped Merlin heal and took him to Camelot but not to the castle, where he warned Merlin to be careful, as "the great battle for Camelot is about to begin". Merlin revealed to Gaius that Morgana was a traitor, and when they saw to Uther his condition worsened as he was tormented by ghosts of Ygraine and several children he had drowned in the Great Purge staring at him. Merlin broke the enchantment by throwing the mandrake root into a fire, while Gaius then gave Uther medicine and left him to recover from his ordeal. The next day, Morgana, having discovered Merlin's survival and the mandrake's disappearance, pulled Merlin aside and threatened to tell Uther that he previously tried to poison her if he told Uther of her and Morgause's plan.
Morgana informed Morgause that Merlin was still alive and broken the mandrake's hold over Uther; however, Morgause assured her that the mandrake had already done its work and by now, Cenred's army was less than a day from Camelot. She also gave Morgana a staff carved from a hawthorn tree on the Isle of the Blessed that was necessary for the final part of their plan. The following day, a patrol of Camelot's knights, led by Sir Leon discovered Cenred's army and raced back to alert Arthur, who finally took charge from the recovering Uther. Leon reported that Cenred's army numbered at least 20,000, outnumbering Camelot's forces two to one. Gaius suggested they negotiate with Cenred, if only to buy them time to prepare, but Arthur, knowing Cenred would not be easily satisfied, ordered Camelot to prepare for a siege, insisting that the castle had never been taken, thereby giving them the advantage. He also ordered that the people of Camelot's outlying villages took shelter within the city. When Leon argued they'd be leaving behind their homes and livestock which Cenred would certainly destroy, Arthur said they'd still have their lives. With Cenred's army only hours away, preparations began in full; weapons and armour were distributed, barricades and defences were erected and the knights of Camelot prepared for battle. As night fell, Cenred's army surrounded the city.
At the urging of Morgause, Cenred began the attack; as catapults bombarded Camelot, Cenred's army broke through the city gates and battle erupted in the streets. Arthur and his men defended the walls and barricades, holding Cenred's men from advancing, and Uther, ignoring the advice of Gaius, Leon and Arthur, donned armour and joined the battle, fighting against the invaders. Arthur pulled him back but he refused to let his kingdom fall, but a crossbow bolt then hit him in the leg. With their king injured and Cenred's men overrunning the barricades, Arthur ordered a retreat back to the castle. As Cenred's men advanced on them, Merlin used magic to block the path with fire, leaving the lower town to the attackers. Cenred stated that taking the castle would be much more difficult, but Morgause assured her ally Morgana would come through and fulfil her side of their bargain. Meanwhile, in the castle, Gaius and Merlin tended to the wounded in a makeshift hospital, where Morgana was also present. They tried to keep an eye on her, but Morgana evaded them and heads down to the crypts with the hawthorn staff where she drove the staff into the stone floor. This awakened the dead as dozens of skeletal hands burst from the floor of the crypt.
As the Knights of Camelot fought a desperate battle to keep Cenred's men from breaking into the castle, they were shocked to find skeleton warriors begin to emerge from the castle and attack them from behind. As Arthur and his men fought a battle on two fronts to hold off Cenred's men and the undead, Merlin headed down to the crypts to confront Morgana. He begged her to stop, but she refused, saying that she had no intention of showing mercy to Uther, as he hunted down all like her. Merlin, not revealing he has magic, empathised with her, insisting that there could be another way and that a gift like magic should be used for good, but Morgana refused to listen. She and Merlin drew swords and duelled, until Merlin used his magic to cause part of the ceiling to collapse, knocking her out. With Morgana incapacitated, Merlin used his magic and his sword to shatter the staff; with the necromantic enchantment animating them broken, the skeleton warriors collapsed into inert piles of bones. With the undead threat defeated, the knights of Camelot returned to the battle against Cenred's men, fighting with renewed vigour. Seeing the battle turning against him, Cenred ordered his army to retreat, ignoring a furious Morgause's warning that she had failed him.
Aftermath
Merlin tried to reveal to Arthur that Morgana was the traitor but she gotten there ahead of him. She told Uther and Arthur that she destroyed the staff and stopped the skeletons, she was falsely praised as a hero who almost single-handedly turned the tide of the battle. After Kilgharrah's riddles to Merlin in the past about what was to come, Merlin recognised Arthur preparing to face the invaders as a sign of his destiny to be Camelot's greatest king and believed Camelot's victory here would be remembered forever to the End of Time. After hearing this from Merlin, Arthur noted to him that he sometimes shown a show of wisdom. (The Tears of Uther Pendragon)