Gul`dan is a fictional male Orc in the Warcraft series. The character was portrayed on screen by Daniel Wu. His allegiance / loyalty is to the Horde. In game lore, Gul'dan was the Orc warlock who open the portal from Draenor to Azeroth with the help of Medivh in Draenor. Although he is mentioned in the past, he becomes the main antagonist in the World of Warcraft - Warlords of Draenor, the fifth expansion of the game. He was corrupted by the Burning Legion, the army that was set up the fallen Titan Sargeras. There is no narration at the start of the film which would have help explain Gul'dan's past and his corruption.
In the movie, he is the main antagonist who has taken control of the Orcs in their home world of Draenor and leads them on their invasion of Azeroth. Durotan remarks that since Gul'dan has taken over the Horde, the planet, once rich in flora and fauna has turned into a wasteland.
Gul'dan is heavily addicted to an energy source known as Fel, it is green in power and its drawn from the living. Gul'dan used captured Draenei as the power source on Draenei and humans on the planet of Azeroth.
Orcs are an honour based civilisation and when Durotan challenges Gul'dan, Durotan looks certain to win until Gul'dan cheats and uses the Fel to gain the upper hand and win. Those skulls in the picture are sown into his skin unlike the other Orcs.
Durotan is killed when Gul'dan sucks the life force from Durotan. All the other Orc's are scared to challenge the power of Gul'dan and submit to his commands.
Although it is not done in the film, in the game, there was a video that was released that showed the history of Gul'dan. Gul'dan was a cripple at birth and struggled. When the other Orcs of the clan turned on him, the leader of the clan told Gul'dan to leave which Gul'dan did. Gul'dan went to the Throne of the Elements in Nagrand. Once at the throne, Gul'dan was turned away by the elemental forces (Air, Earth, Fire, Water) and instead was consumed by Fel.
Gul'dan would return to his village and kill every single person in the village in revenge for what they had done to him. Guldan's Origins
Gul'dan would lead the Orcs from Draenor through the Dark Portal to try to conquer Azeroth using Fel, a magical force that is harvested by the living energy of living people. In the game, Gul'dan is the primary antagonist in Warlords of Draenor expansion where he manipulates the Orc chief to go through the portal. When the Chieftain decides to stay in Tanaan Forrest with his Hellfire Citadel, Gul'dan wreaks revenge on the chief.
Gul'dan would travel into his future, to the time of when the World of Warcraft is set, after the invasion. Gul'dan would discover Illidan Stormrages' remains and set about trying to revive Illidan. Gul'dan's journey would take him to Suramar City, a city of Nightborne Elves on the Broken Isles. The Broken Isles was a continent that was cut off from the rest of the world. Gul'dan's attempts to recover Illidan would backfire as Illidan would kill Gul'dan by crushing his skull.
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