Overlord tried to seize control of the Multi-verse, and everything in it. When faced by his fellow guardians he ended up lossing. He was locked into a special crystal and sent to the end of the Universe.
After millions of years he finally escaped and headed to kill the people that inprisonment. He found his first victim on Planet New Namek, Ultimus. After Ultimus declined he offer to join him in total domination, he easily defeated him. He found Guardian on earth and once again he easily defeated him. He found Rampage, well actually Rampage found him after Ultimus told Rampage. He had a tuff fight with Rampage, but in the end won again.
Wise who had been watching from his crystal ball, decided that only Goku and Vegeta could defeat Overlord. He teleported all of them to the Sacred World of The Kai's, that way no planets would be destroyed. Wise also tried to fight with him along side Goku and Vegeta. Overlord only drained his life force to mear nothing. This gave him more power then he ever imagined. Goku as an Ultimate Super Saiyan and Vegeta as an Eternal Super Saiyan were unable to even scratch him. They fused into Ultimate Eternal Super Saiyan Vegito. They went head to head with Overlord and still were unable to beat him. Overlord then absorbed Rampage's and Ultimus's power, letting him completly dominate over Vegito. He charged his final attack, Omega Galactic Blast. Vegito and him locked hands and tried to get the other to surrender, but instead of Vegito beteating him, he split back into two from absorbing to much power from Overlord. Alpha Goku and Alpha Vegeta were created. They easily overpowered Overlord and defeated him with a combo of Goku's Eternal Kamehameha and Vegeta's Infinite Final Flash.