When Babidi attempted to mystically take control of Vegeta, he invaded his mind, something he could only achieve because of the hidden depths of evil lurking within his heart. This action was Babidi's last defence in an effort to use him against the other Z Fighters. It is revealed that Vegeta could have stopped his possession but willingly chose to let Babidi take control of his body, believing that by awakening the evil intentions within himself he would finally be able to defeat Goku. As he fought along side the Z Fighters during the years and grew attached to his new home on Earth and his family, Vegeta struggled to hold onto his merciless dark side which he believed was the true source of his power.Vegeta realized that without a heart of pure good or pure evil, competition with Goku, and after being saved by Gohan during the Cell Games, that he was slowly losing his power. Seeing how Babidi's magic had affected Majin followers, Vegeta realized that Babidi's magic could return him to his former self, as he had been when he first arrived on Earth and fought Goku.As Majin Vegeta, Vegeta is a Super Saiyan 2, his eyelids are black, and the "M" symbol on his forehead, the mark of the Majin, indicating he is now a servant of Babidi.
During his brief incarnation, Majin Vegeta uses his possessed state to act ruthlessly and without regard for the lives of others, killing indiscriminately to force Goku to fight. Over the course of the fight he completely overrides the mind-control of Babidi with sheer will alone. Majin Vegeta reveals the bitterness he has held within himself all the years that Goku has been deceased and that he had been forced to live with his loss of pride, the only thing he believes has any value.
Eventually, Vegeta knocks Goku unconscious so that he can fight the newly released Majin Buu alone and prove he does not need Goku's help. But, despite his efforts against Buu, Majin Vegeta cannot overcome his endless stamina and regeneration. As his final selfless act, Majin Vegeta self destructs in order to save Bulma, Trunks, Goku and the entire world from the threat of Majin Buu, despite knowing that unlike Goku, he wouldn't keep his body after death. This marked the final transformation of Vegeta from his selfish evil and neutral state into a good hero. Upon death, he is finally freed from the symbol of the Majin and wished back a short time later.
He later reappears in Dragon Ball Z: Shin Budokai - Another Road, as Future Babidi again uses his magic to try and brainwash Vegeta. However, unlike the last time Vegeta transformed into Majin Vegeta, this time, it was completely against Vegeta's will. He also appears in Dragon Ball Z: Shin Budokai in a similar manner, in that Vegeta does battle with Majin Vegeta (in reality Janemba in disguise), but Majin Vegeta/Janemba then possesses him, despite Vegeta's best efforts to prevent it.
While having the Majin seal on his forehead, Vegeta gains substantial augmentations in power, speed, strength, and endurance. He can use some attacks, only available and/or also he can use in this state. While in the Majin state, Vegeta's signature attacks, such as Big Bang Attack and Final Flash, have greater attack power and destructive force.