Dark Oak, also known as his real name as "Lucas" is the powerful and tyrannical leader of the Metarex, coincidentally the antagonist of the Metarex arc of Sonic X.
Many years ago, Dark Oak was originally born Lucas, a male member of a plant-like species and at some point married Earthia. Eventually rising to the combat faction leader of Green Gate, Lucas and his forces entered a war with an unknown enemy of animals when his planet was ensnared in the turmoil of war. As they were losing the battle, Earthia wanted to leave their planet behind and start a new life, retreating from their enemies. Being a warrior, Lucas would rather stay and fight to stop the enemy causing further destruction in the universe, which brought him into conflict with his wife. Eventually, Earthia told him if he did not like it he could stay and fight the enemy himself and Lucas agreed.
Following that, Lucas and the majority of males in his race stayed behind to fight. Knowing that the planet had been abandoned, Lucas extracted his home world's Planet Egg and tried to harness its energy to sustain his and his followers' monster forms without dying. His actions were discovered by Earthia, who had discovered the plants were dying before leaving the planet. Horrified that her husband had sacrificed so much to fight, Earthia called Lucas a traitor for destroying his own world to fight the enemy, and Lucas called Earthia a traitor for wanting to abandon their planet entirely. Earthia and all the female members of the race left while Lucas and his fellow soldiers remained behind to fight for their lost cause.
Fearing the lengths her husband would go to end war, Earthia ordered the remaining males to be killed and unleashed a nuclear strike that devastated their world. Lucas and the few that survived swore revenge against this betrayal. Perfecting their science, they used the Planet Egg to create powerful cybernetic bodies for themselves and vowed to end all conflict in the universe by eliminating all life except plants and plant-based creatures. Now referring to themselves as the Metarex, they took new names for themselves and raised an army. From this point onwards they crusaded across the universe, harvesting the Planet Eggs they required to achieve their final goal. At some point afterwards they tracked down Earthia’s space cruiser and on Dark Oak’s orders, slaughtered everyone onboard, sparing only his daughter Cosmo.
Dark Oak first appeared to challenge Super Sonic in orbit around Sonic's world in order to claim the Chaos Emeralds from him. Despite Super Sonic's powers, he was unable to defeat Dark Oak. Dark Oak taunted at Sonic that his defeat was certain. Surprisingly, Sonic used Chaos Control to send the emeralds away before he fell towards his home planet. Dark Oak remarked that Sonic had simply delayed his triumph. Before Dark Oak departed for space, he alerted his forces to search for the Chaos Emeralds and to send a Metarex to remove the Planet Egg of Sonic's world.
Throughout the series, Dark Oak appeared behind the scenes, scheming his enemies' downfall in secret. He dispatched Scarship, the space-faring Metarex, to destroy Team Sonic. 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 containing the last two Chaos Emeralds was destroyed and the jewels scattered, Dark Oak declares that it was only a test. From that point on, Dark Oak stays out of the action, dispatching his Metarex commanders in various attempts to kill Sonic and co. He also recruits Doctor Eggman as an ally, though Eggman later proved traitorous.
Dark Oak's plan to let plant life reign supreme was later revealed to occur at galactic coordinates 00,00,00. Every 1,000 year period, a time/space anomaly occurs wherein the “Galaxy Corridor” will align so that all points in space connect to Planet Aqurius. This would mean that whatever events were to happen there would be able to spread across the universe. Using the Planet Eggs as a source of water and the Chaos Emeralds as a source of light, Dark Oak combined with his last two commanders to form Final Mova. Knuckles commanded the Master Emerald to give all its power to the Sonic Power Cannon, which fired a powerful energy blast that killed two of the three heads that the beast had and depowered the Chaos Emeralds, but Dark Oak was still alive and had already extracted all the energy he needed from the emeralds. Dark Oak attempted to finally achieve his goal by entering Final Mova’s second phase, where he would then drain the life out of all animal and robotic creatures and attempt to have them consumed by endless forests - a feat he claimed would allow all life to live forever in complete peace, silence and stillness as all animals are assimilated into the plantlife. However, being plant-based in nature, Cosmo was not affected. She was able to give the restored Chaos Emeralds to Sonic and Shadow, allowing them to assume their Super forms. They attempted to thwart Dark Oak’s plan, but only succeded in spurring him into further rage. Seeing that his plan had been foiled and intent on taking his enemies down with him, he entered Final Mova’s third phase in the shape of a massive, shadowy planet and created a powerful gravity field (which even Super Sonic and Super Shadow couldn't break through), planning to crush the Planet Eggs, causing it to explode and destroying everything in the entire universe for having rebuffed his offer of peace. Cosmo, in a self-sacrificing act, fused with Dark Oak and was able to immobilize him with her power. Reluctantly, Tails fired the Sonic Power Cannon with Super Sonic and Super Shadow in it, allowing the team to kill Dark Oak once and for all. Super Shadow then used Chaos Control, removing himself and the remaining concentration of energy far away from the vicinity before they could erupt, thus destroying Final Mova.
Following Dark Oak's destruction, Lucas was shown in a mysterious location, greatly saddened by his long and futile efforts to end conflict. In the English dubbed version, he ponders his actions, now regretful and wishing to repent. There, he was greeted by and departed with a form of Earthia, having forgiven each other of their pasts.
In the Legacy of Chris Thorndyke, he is a member of the Council of Apocalypse after being found by Tokoyami and offered a deal he joins and he will get a piece of the Omniverse and he agreed and considered one of the Overarching Antagonists of the Sonic Series.
And he is voiced by Crispin Freeman as Dark Oak and reprised by Matt Hoverman as Lucas in the English Dub and is reprised by Jouji Nakata as Dark Oak and Katsuyuki Konishi as Lucas in the Japanese dub.