Dark Oak is the true main antagonist of Sonic X, more specifically during the final season. He is the powerful leader of the Metarex, an army of bio-robots that scours the galaxy for the Planet Eggs, jewels that contain the life energies of their planets.
Dark Oak has one large, green eye and carries a golden sword. His armor is composed of varied shades of purple, with a large green orb on his chest and two tattered halves of a black cape hanging from his shoulders.
He is by far the most powerful of the cyborg aliens whose power surpasses even that of Super Sonic's, but his abilities are mostly unknown since he's never done much fighting onscreen. Aside from his weapon, all that's known about how he fights is that his speed is at-least on a level comparable with Super Sonic's. He is an intelligent schemer, but he never seems to take defeating Sonic and his friends seriously enough to fight them himself.
Dark Oak first appears to challenge Super Sonic in orbit around Sonic's planet. Despite Super Sonic's seeming invincibility in his previous battles, he is unable to defeat Dark Oak. In order to prevent Oak from stealing the Chaos Emeralds, Sonic uses Chaos Control to send them away. As Sonic falls toward the planet, his power drained, Oak departs for space, revealing injuries Sonic hadn't noticed. His final act in that incident was to alert his forces to search for the Chaos Emeralds and to send a Metarex to remove the Planet Egg of Sonic's world. Later, Oak appears behind the scenes, scheming his enemies' downfall in secret. He dispatches Scarship, the spacefaring Metarex, to kill them. After this fails, Dark Oak confronts Sonic and co. in a battle over the seven Chaos Emeralds at one of his fortresses, but they soon find that he was simply contacting them with a hologram. After the fortress where he was keeping the last two Chaos Emeralds was destroyed and the jewels scattered, Dark Oak declares that it was only a test.
Amy Rose and Rouge the Bat later discover the true identity of Dark Oak and the origin of the Metarex through a holographic account left by Cosmo's mother Hertia. Dark Oak was once a male member of Cosmo's species named Lucas and their planet was ensnared in the turmoil of war. Hertia wanted to leave the planet behind and start a new life, retreating from their enemies. Lucas didn't agree and did not want the soldiers to die in vain. Hertia/Earthia told him if he didn't like it he could stay and fight the enemy himself, and Lucas agreed. It is implied that he maybe Cosmo's father, though it is confirmed.Later Hertia was alerted to the plants outside her palace dying and came to find Lucas stole the Planet Egg from his own planet to try and harness the power to attain their "beast form" for longer periods of time. Hertia called Lucas a traitor for destroying his own world to fight the enemy, and Lucas called Hertia a traitor for wanting to abandon the war entirely. Lucas vowed to crystallize the life force of all planets to build an army to combat their enemy and Hertia left him and his fellow rebels to fight for their lost cause. With a final act, Hertia ordered the remaining beasts killed along with the planet, and unleashed a nuclear strike that devastated their world. But unknown to her, Lucas survived, and he and his rebels became the Metarex. It was then that they vowed to fulfill the destiny of the universe by eliminating all life except plants and plant based creatures. His plan to do this is later revealed at galactic coordinates 000. At the end of a 1000 year period, Dark Oak arrives in time for a time/space anomaly to open. Using the energy released from an alternate dimension, he begins to construct a massive plant which will aid in the Forestation process. Using the Planet Eggs as a source of water and the Chaos Emeralds as a source of light, the two things most essential to plant growth, Dark Oak combines with his two most trusted commanders, Pale Bay Leaf and Black Narcissus, to form a dragon-like plant entity with three heads known as Final Nova.He then absorbed all the power of the Chaos Emeralds, and supposedly grew even more powerful. Sonic loaded himself into the Sonic Driver, and killed 2 of the 3 heads that the beast had, but Dark Oak was still alive. Cosmo's mother, Hertia, told her that this was her time to act. Though Dark Oak made an attempt to suck the life out of everyone in order to make himself stronger, Cosmo was not affected because she and Dark Oak both came from the same species. She was able to give the Chaos Emeralds to Sonic and Shadow, allowing them to assume their Super forms. They seemingly killed Dark Oak, but again he survived, albeit narrowly. Realizing that he was going to die, he created a massive gravity field , threatening to cause the Planet Egg to explode, destroying everything in the entire galaxy, intent on taking everyone down with him. Cosmo, in a self-sacrificing act, fused with Dark Oak and was able to immobilize Dark Oak with her power. Reluctantly Tails had no choice but to fire the Sonic Driver with Super Sonic and Super Shadow in it .Sonic attempts to use Chaos Control, but Shadow suddenly punches him in the stomach, causing Sonic to faint. Shadow then uses Chaos Control himself removing himself and the Planet Egg from the vicinity. He disappears and is presumed dead by everyone, but considering Shadow's immortality and frequent habit of almost dying and then returning, it is unlikely. The combined power of the hedgehogs killed Dark Oak once and for all, and peace came to the galaxy once more.
With Dark Oak's defeat, Lucas was shown in an unknown location, reformed and repentant. A vision of Hertia appeared before him and he departed with her for the afterlife together. It's unknown what became of Lucas after that, but it is likely that he is happily reunited with Hertia.
- Although Dr. Eggman drove the plot in the Sonic X series throughout the first two seasons and mostly in the third, Dark Oak is the true main villain in Season 3 and the overall main antagonist of the series itself because he had bigger plans than anyone else (including Eggman and the Metarex).