The Darkest Saiyan is a permanent fusion between two of the most powerful characters in Dragonball. Goku and Vegeta are ultimately fused together forever and Dark Vegito is from an alternate universe where their fusion cannot be broken apart. Due to this, he is one of the strongest Saiyans in existence. This also created much confusion when he returned home, due to the fact that he had feelings for both Bulma, and Chi-chi. However, since Chi-Chi had grown accustomed to Goku never really being home, she told him to continue his relationship with Bulma and broke off their connection. However she also asked him to continue being a father to his children that they had together, to come back from time to time to spend time with them and he agreed to do so.
After a few years, Bulma and Dark Vegito had a daughter together, named Bra. Due to the combination between both Vegeta's and Goku's genes, Bra had potential far stronger than any typical saiyan. It's said she was born with a power level of 13,460.
A few more years later Dark Vegito found out that an evil scientist took dna from him and went out to create another Dark Saiyan. The Dark Saiyan came to him and announced that he was his son, and that he was initially created from his dna and the dna of an unknown deity. He gave his name as Xicor, and stated that he was potentially in an eternal state of Super Saiyan 5.
Xicors power easily rivaled that of Dark Vegito's, and even surpassed it, and in order to defeat Xicor, Vegito had to call upon the power of the planet and the inhabitants themselves to borrow enough strength to use a spirit bomb powered dragon fist.
However this did not ultimately destroy Xicor, but left him in a coma state. Wanting to get to know his nee son, Dark Vegito gathered the dragonballs and wished for Xicor to revive, but he also asked the dragon to remove his evil intentions and replace them with good. Being within the power of the Dragon, this wish was easily granted and Xicor awoke, clear of the darkness inside of him.
Happy to finally be fear of his evil thoughts, Xicor grew close to Dark Vegito and his new sister Bra. Little did they all know, the darkness that once consumed him, took on the shape of a new foe.
The pure evil that once was inside Xicor, took on a sinister manifestation of its own and seeing how powerful the saiyans were, it modeled itself after them, creating itself a body of a saiyan.
Just like Xicor, this evil was eternally stuck in a form of Super Saiyan 5, and held incredible power. However it also contained the ability to corrupt others, and to turn them against both Xicor and Dark Vegito.
Father and son fought hard against this foe, until Dark Vegito himself was consumed by the darkness and became an enemy of Xicor. For some reason, probably due to the wish his father bestowed upon him, Xicor was completely immune to the darkness and within him held the key to destroying it forever.
Taking up refugee within the hyperbolic time chamber, Xicor became stronger and found the link to the purifying power within himself. However he didn't hvae much time in the hyperbolic time chamber, before the evil itself found a way inside.
The battle of dark vs light began, and struggling through the fight, Xicor was almost defeated, but before the last blow could be struck, the light within him bestowed upon him a new power and a new confidence.
Lashing out with all the power he had, Xicor managed to completely erradicate the darkness, and freed those who were under control.
Weakened and fatigued, Xicor fainted from his efforts and remained asleep within the hyperbolic chamber for days, until his father entered the chamber and brought him out. The strain from his fight had almost killed him, and had his father not brought him out of the chamber and gave him a sensu bean, he most surly would of died.