"TOO LATE! AIN'T NOBODY GONNA STOP THE BEAST NOW!" - Xevin transforming into a Legendary Super Saiyan in the Universal Survival Tournament
Legendary Super Saiyan is similar to Super Saiyan, however the user's power is much higher and their speed is also increased, making it a stronger form of Super Saiyan.
Currently only three people in the known Multiverse have wielded this incredibly powerful form, Vex, Kaneki and Xevin. There are only two ways to achieve Legendary Super Saiyan. You must either be born with the power, or share the same blood as someone who has the power.
History of Legendary Super Saiyan
Vex is the first one we've seen use this form. Before transforming he felt oddly ill to the point where he couldn't even control his own power. He was taken to a random house in the wilderness and was being taken care of by Ocari while the others went to Capsule Corp, looking to find out what was wrong with Vex.
During this time Vex's muscles grew twice their size and his rampage as a Legendary Super Saiyan had begun. He had almost completely destroyed the forest trying to kill Ocari, and during this time Ken and Choken had returned sensing the enormous power trying to stop Vex. Eventually Vex had calmed down and returned to his normal self.
While Xevin's first time transforming into a Legendary Super Saiyan is unknown, his first time in the series was against the defenders of Planet Tenja, Otani, Aisu, Gani, Tsuke, and Strike. While the war was taking place between Xevin's men and the citizens of Planet Tenja, Xevin fought the Heroes, and while the Heroes were already losing, Xevin decided to try and cement his victory by turning into a Legendary Super Saiyan, causing chaos all across the planet, until he was eventually defeated shortly after Gatani defused and Scargo sacrificed himself.