Majin Goku was created when the deceased Babidi worked his evil magic on our hero. After a prolonged mental battle, Goku succumbed to the taxing mental toll on his body- creating Majin Goku. This form is much different from any Saiyan's form. Goku's Super Saiyan powers are completely revamped, but nonetheless much more powerful. This form is defeated by Goku's great-great grandson, Goku Jr., after the latter transformed into SSJ 3 . After defeat, Goku reverts to his normal form.
"Do you remember how badly you wanted to battle me when you were a Majin, Vegeta? Well, now I share the same thrist for fighting, and you WILL GIVE IT TO ME!!!"
— Majin Goku to Vegeta
Tired of being trapped in Hell forever, Babidi decided to free himself. But with no other options left for him, he turned into his special power to make people Majins . After pondering through a list of heroes to transform, he settled for the best fighter, Goku. Babidi took over Goku's mind, but Goku's mental powers were far more than Babidi's. However, the past villains of DBZ gave their power to Babidi, allowing him to take over Goku's mind. After a defeaning scream and a grey flash, Majin Goku was born. However contradict to his master's thoughts, Goku wasn't completed in his control and didn't obey his orders to free him from Hell. Instead, the new powered up Goku sent a mental shockwave to Babidi, blowing up his head (for the second time). His Saiyan instincts took over and he immediately teleported to Vegeta, asking for a death match. Vegeta realized with horror, Goku has suffered the same fate he had before and tried to reason with his Saiyan friend. However, Goku was in no mood to reason and threatened to destroy his former beloved planet. Fearing for the lives of his loved ones, Vegeta teleported him and Goku into a faraway planet to begin their duel.
"Now is the time to see what YOU'RE REALLY MADE OF AGAINST ME!"
— Majin Goku to Vegeta
Vegeta and newborn Majin Goku clashed into an epic battle. But with horror (and with Saiyan interest), Vegeta realized Goku's new Majin powers far exceeded his. Vegeta followed his reasoning up with a Super Saiyan transformation. He resumed his battle and gained a quick upper hand, however Goku was still not brutally outmatched. Vegeta was shocked and said that a SSJ should've crippled Goku and followed up with a ASSJ transformation. Only then did he fully take control of the battle, crushing Goku into oblivion. However, fuled by new Majin rage, the Saiyan transformed into Super Saiyan. With two crushing blows, he almost vanquished Vegeta. However Vegeta caught Goku off guard, transforming into Super Saiyan 2 and blasting Goku through the heart with Ultra Final Flash. Vegeta cockily exclaimed, "Well Kakarot, even with that new strength, you're as stupid as ever!" Goku proved Vegeta wrong by easily getting up, although battered. However Goku's strength was matched evenly by Vegeta's wit, putting the one-sided battle into a stalemate. But Goku uses Kaio-ken and brutally tosses Vegeta into a fiery ocean, seemingly obilerating the Saiyan Prince.
But a bright yellow flash filled the ocean and it erupted and out came Super Saiyan 3 Vegeta. The new Mega-powered Saiyan easily took control of the battle, crushing Goku. Goku fired Majin Kamehameha, but Vegeta simply tossed it away, and retured with a Mega Galick Gun, forcing Goku to return to his normal state. But Goku still stood up, and brilliantly transformed into Super Saiyan 2 Majin Goku. The two battle evenly, but Vegeta took the lead sometimes with his natural SSJ 3 advantage. They sparred like this for awhile, slowly tearing away the unlucky planet in result. Vegeta realized he was loosing energy at a more rapid rate than Goku because of his Majin qualities. Vegeta decided to solve that by transforming into SSJ 4. Vegeta immediately took over pounding Goku. He started beating Goku so hard, his Majin form was shimmering away. Vegeta realized that killing Goku will eventually wash away the evil from him. He was about to unleash an Omega Final Shine when Goku transformed once again, reaching his fearsome SSJ 3. Vegeta and Goku appeared to be even for awhile, but then Goku showed off his true power, stunning Vegeta and overpowering him immensly. Goku then started blasting him with a Mega Full-Power Energy Wave Volley, and Vegeta started reverting to normal from the excess damage. Goku said how he always wanted a battle with Vegeta ever since the defeat of Omega Shenron to see how much they had improved. But Goku's good heart wanted to protect Vegeta and the sake of the universe and he simply assumed that Vegeta obviously got stronger. But now his pure evil heart decided against assuming and wanted the ultimate duel no matter the cost of the universe or the innocent. With this Goku prepared a Mega Dragon Fist to end the stunned Vegeta's life. As he closed in for the kill, a beam landed right in his eyes, blinding Goku. The furious Saiyan looked around for the source and he found an unlikely one, Goku Jr!!
"You are promising child... But sadly I have to kill you..."
— SSJ 3 Goku commenting on SSJ Goku jr.'s strength
The Saiyan rushed to Vegeta's side and the Saiyan Prince explained the seriousness of the situation and handed over the responsibilities of the universe in the young boy's hand, saying his training has made him far stronger than Vegeta himself. Goku Jr., healed Vegeta and then looked at his ancestor, ready for battle.
Goku insults his descendant, saying a little weakling can't possibly protect the universe. However, Goku Jr says his training from all around the universe has made him surpass any individual in strength, except for his long-time comrades. Goku simply snickers, but the young Goku transforms into Super Saiyan, eager to show his ancestor/mentor/enemy a test of his own medicine.
The two engage in combat, and the adult Saiyans are deeply suprised to see a young Super Saiyan matching evenly with an experienced SSJ 3. Goku Jr dishes out some fancy attacks and Vegeta complements them all. Goku says he is extremely impressed with his great-great-grandson and asks him to join forces. But Goku Jr refuses, saying that when he's through with Goku, his beloved grandfather would be restored. Goku smiles and powers up, revealing he has been hiding his power. Goku Jr is badly beaten by the new SSJ 3 and he transforms into SSJ 2, displaying abilities no Saiyan has done before.
The new SSJ 2 Goku Jr fights Goku and overtakes him, shocking Vegeta. The evil Goku uses his Majin tricks to overtake Goku Jr, by the mind but the kid refuses to listen pounding Goku Sr. Goku Jr says this his ultimate form and there's no way getting past the mounmental power of his. Goku smiles and gets up, saying he has made him proud and it'll be sad to kill him, displaying affection for the young boy, even at Majin state. He then powers up even more, and in a red and grey light, he transfoms into Ultimate SSJ 3.
The newly transformed Goku faces off against Goku Jr, brutally overtaking the child. Vegeta notes this is Goku's most powerful and malicious form in his Majin state. This is confirmed when Goku shows no care for his grandson, and bloodily beats him. Vegeta- not taking more of the beating- powers into SSJ 4 and charges at Goku. But the evil Saiyan blasts Vegeta in the face with a Kamehameha without even looking, almost killing Vegeta. Goku thinks he's done with playing around with "these two helpless idiots" and takes flight. He then powers up a Ultimate Majin Kamehameha to destroy the whole galaxy. As he's powering up the attack, Vegeta tells how Goku will destroy the whole universe and how all the innocent people will be obliterated. Moved by his speech and driven by the force to save the innocent, Goku Jr initates a transformation into the next form...
— SSJ 3 Majin Goku commenting on SSJ 3 Goku Jr.'s strength
Goku Jr roars as he transforms into SSJ 3. A bright flash reveals a Goku Jr, sparkling with SSJ 3 power around him. He declares he's going to defeat Goku if it's the last thing he does. Goku simply snickers and fires his attack. The red blast zooms in on Goku Jr, threatening to destroy the galaxy. However, Goku Jr simply stops the devilish attack with his hands, stunning Vegeta and Goku. He then absorbs the energy and converts it into positive energy and distributes that energy among the battle-worn planet, restoring it. He then engages Goku in a fierece battle, tearing up the whole universe in process. Goku repeatedly beats his ancestor, but is unable to bring himself to kill him. Because of this, the evil Goku beats down the boy. Plus, no formal training in SSJ 3 form begins to wear Goku Jr down fast. After beating around the weakening kid, Goku decides to push his luck by attempting to kill Vegeta. Goku Jr saves the Saiyan Prince's life at the last minute, and Goku's evil act finally pushes him over the curb. He powers up and fires a Standing Ultimate Kamehameha. The blast hits Goku full force and the Majin in our hero is completely oblierated. Goku Jr then collapses from exhaustation, and Goku and Vegeta share a smile, nodding that the universe is safe because of the next generation.