This may come as a surprised to a common observer, but Korran wasn't always the monstrous beast, he is today. At one point in time, he had been a Skakdi and a pretty well known one at that considering he was the Piraka Vezok. So, obviously he has most of the background of that of the main universe Vezok with only a few notable details being different than that of listed.
For starters, somewhere along the line during either Vezok's time with the Dark Hunters or his time as a thief, he ran into the future hero of Metru Nui, Veef; where he developed a sort of respect for the Toa enough to considered him a friend at the very least. And then there is the detail of his and other Piraka arrival on Voya Nui, which differs slightly from the main universe's storyline being that sometime after they defeated the Toa Nuva,The Forbidden One forcefully pull them from their time period to the present or rather the BK present and due to methods used to do this, the Piraka had been mutated into what they are today and the Forbidden One was also physically scarred permanently as a result.
After they were pulled from their time period, the Forbidden One had studied them each and designed cages within the Coliseum to hold them at bay until he needed them. For Korran, he was placed with a large tank of water imported from Voya Nui due to an unfortunate side effect in his mutation causing him to needed the conditions of that Voya Nui's surrounding waters in order to survive for sometime until his new body was adapted to its new environment. And as an extra safety precaution, the Forbidden One install chips within each of the Plagues to both ensure the Plagues obey his commands and also to serve as a kill switch should they go rouge.
The chip prove to be quite efficient on Korran, when the Forbidden One first release him to distract the Toa rebellion from halting his plan to bring the Toa Nuva back to life. However because of this move, it had set the course for Veef, who was somehow friends with Vezok in his Skakdi days, to attempt to free his old friend from the enslavement of the Forbidden One. It took some work but Veef was able to fried the chip install within Korran and being so grateful to Veef for doing this, he teamed up with him to try and find the Book of Chronicles that would send the Forbidden One reign over the city to quick but final end. The duo were successful in doing this, thanks to the help of both the Last Chroniclers and the Lightkeepers.
Unfortunately for them, they had learned the truth too late to stop the Forbidden One from unleashing his plan to bring back the Toa Nuva for he had both release a herd of cloned Kanohi Dragons and the six other members of the Plagues into the city with the hope that his manipulated of time would cause the Toa Nuva to return for good to stop the threat on the city and the plan work at least for a minute or two before the Toa Nuva literally became dust (Expect for their Kanohi.). Luckily for the city, the Plagues started to handle the cloned Kanohi Dragons due to them all being extremely hungry, so they had chase them across the silver sea to the land of Karzahni, which, due to the return of original six elements cause by the Kanohi Nuva left behind by the now dead Toa Nuva,had its gates reopen. (TBC)
- It should be noted that it is likely that the method used to extract him from the Voya Nui era along with the other Piraka had merely created a copy of the Piraka's minds that was pull through time to present by the Forbidden One, which would mean that Piraka still exist on Voya Nui during the Toa Nuva's Era as a result ensuring that what occurred to make BK present remain intact. It is also likely that the Forbidden One could of took the Piraka in the BK Universe from another alternative universe, so it really depends on how one looks at the situation.