Mordred is a major antagonist on the BBC show Merlin.' He was portrayed by Asa Butterfield as a child and Alexander Vlahos as an adult.
Mordred is first introduced as a young druid boy who is visiting Camelot with his master. After his master is arrested for being a magic user, he calls out to Merlin telepathically in order to obtain his help. Merlin arrives, helps him escape, and takes him to Morgana, who helps Merlin hide him from Uther. It is soon revealed that he had been injured during his escape becomes sick from infection as a result. Merlin, confused on how to deal with the boy, asks the great dragon for his advice. The dragon reveals a prophecy that Mordred would be the one to kill Arthur in the future and advises Merlin to let the boy die. After the execution of his master, Merlin and Morgana convince Gaius to help cure him and they decide to help him escape. After being caught by Arthur Pendragon, the young knight decides to help Mordred in his escape. Merlin chooses to ignore the dragon's warning and the second escape attempt proves successful and Mordred is returned to the druids. He reveals his name and thanks Merlin and Arthur for their assistance before disappearing into the forest with his fellow druids.
Mordred is not seen again until season 2 when Morgana, who is quickly discovering she has magic, goes to the druids in hopes that they can help her control her abilities. It is there that Mordred and Morgana are reunited and the young druid convinces his people to help him take care of her. Later, Uther orders his soldiers to attack the druid camp (under the impression that they kidnapped Morgana) and Morgana is taken away back to Camelot. During these events, Merlin reunited with Mordred and sees him kill two soldiers with his magic before escaping. It is then that Merlin begins to take the great dragon's warning about Mordred more seriously.
Mordred again appears in another episode where he has sided with a group of bandits lead by Alvarr and the two convince Morgana to steal the Crystal of Neahtid, which she does. After being reminded of the prophecy in which Mordred would slay Arthur, Merlin goes to the bandit camp and helps massacre all the members except for Alvarr, who is captured and later freed by Morgana, and Mordred. Merlin does attempt to indirectly kill an escaping Mordred by tripping him with his magic and letting two soldiers kill him. The plan fails, however, when Mordred kills the soldiers with a couple of spears. Mordred then sees that Merlin was the one who tripped him and telepathically states to the warlock: "I shall never forgive you Emrys, and I shall never forget." He then escapes into the woods and isn't heard from again.
Mordred doesn't appear in the show again until season five. Set many years after the events of season two, Mordred is now an adult who has left the druids and has allied himself with a group of renegades led by Ragnor. He is then reunited with Merlin and Arthur for the first time in several years when they are captured by his allies and are about to be killed. He prevents this from happening by convincing Ragnor to take them to Morgana, who is now an evil sorceress. While they are taking the two to Morgana, he reveals his identity to them, much to their shock and states that he owes Arthur his life after he helped him escape Camelot as a child. Merlin and Arthur manage to escape from the group and spare Mordred's life while doing so. He and Ragnor then report to Morgana, who recognizes him and acts kindly towards as a result, and tell her that the duo escaped, much to her displeasure. Morgana, accompanied by Mordred, then tracks down Arthur and tries to kill him while Merlin is incapacitated. Mordred, in order to repay his debt, then stabs Morgana which allows Arthur and Merlin to escape with their lives. He is then taken back to Camelot and is knighted. He later visits Merlin and takes back his threat from many years ago stating that Arthur is good and that Merlin can trust him. Regardless, Merlin remains wary of the young knight.
Mordred then serves as a knight of Camelot under the now King Arthur and accompanies the Knights of the Round Table (and Merlin) on numerous adventures throughout the season. Throughout the season, Merlin struggles to truly trust Mordred and often asks Gaius for advice about the magical knight, who often responded with uncertainty. This continues until a girl named Kara arrives in Camelot and is wounded during a hunting accident by Arthur. Mordred recognizes the girl as an old friend and tries to help heal her injuries. Merlin, growing suspicious, asks Mordred what he's up to and the knight then reveals everything to him while asking that it be kept a secret. Merlin betrays this trust and tells Arthur about the girl, who is arrested after trying to kill Arthur and is sentenced to death. Mordred begs the king to spare her life and is refused many times. As a result, Mordred tries to help her escape but is stopped by Merlin and the Knights of the Round Table. They are both arrested and Kara is executed via hanging. After this occurs, Mordred angrily escapes the dungeon using magic.
Mordred swears revenge on Merlin, Arthur, and Camelot and then reports to Morgana, where the two decide to become allies once more. Mordred also becomes the one to reveal Merlin's true identity as Emrys to Morgana, thus making Morgana and Merlin archenemies. They then both use the Saxon army to wage war on Camelot. It is soon revealed that Arthur is leading an army to intercept the two and Mordred plan to lead an attack against the army. Morgana then gives Mordred a sword forged with dragon's breath to kill Arthur with and send him and the army on their way. During the ensuing battle, Mordred kills many Camelot soldiers and finds Arthur in the chaos. The two engage in battle and Mordred emerges the victor after fatally wounding Arthur. After asking for forgiveness and understanding from a dying Arthur about his recent actions, the king suddenly rises and stabs an unprepared Mordred in the heart, killing him. As he falls, Mordred smirks at Arthur knowing that the king will die and that he has obtained his vengeance despite his death. After Merlin kills Morgana, Arthur dies in Merlin's arms and the prophecy the warlock tried so hard to prevent came true as a result. However, the great dragon later tells Merlin of another prophecy - that Arthur will rise again one day and once again reign as king.
- He is based off the Mordred character from Arthurian Tales, who was also responsible for the death of King Arthur.