Leiknir was born in a coastal village of Rundwall decades before the Third Great War, and grew up to become a fisherman. He also met his future wife at the village and settled down with her, but some time later, a storm destroyed his boat and they were forced to move to Strand to make a living. Luckily for them, his wife's brother allowed them to live with him at his house there, which left Leiknir with enough money to buy a new fishing boat and supplies. The fishermen already operating there however didn't want more competition and sabotaged his boat, which he only found out when he rowed it into Denglr's Bay.
Although Leiknir survived, he was told by the other fishermen to never come back to fish again. This caused him to grow despondent, since he was now destitute, and with no means to earn a living, he decided to take revenge by killing them and drunk heavily that night. While wandering the docks in his stupor, he noticed a few longships unloading illegally captured slaves at the docks. He thought of going to the authorities, but knew it would not change his situation. He started wandering again while wondering what to do, and soon noticed a ship, the likes of which he had never seen before, and saw its crew unloading slaves as well.
The next morning, Leiknir approached the captain of the strange ship he saw, Magnus, and told him about the other slavers he had realized were operating without his knowledge, in hopes of getting rewarded. Magnus had his Vak'auga gang members verify the information and also learnt from Leiknir as to why he was helping him. After finishing off the other gang, Magnus asked Leiknir why he should let him live, and the latter replied that he could act as his spy since no one noticed him coming and going. Magnus found wisdom in his words and was convinced. He then helped him learn how to navigate Strand, and sent him off with a sack of gold as reward.
Leiknir continued to work for Magnus as his spy and helped him with his illegal business of smuggling and slavery, but eventually started to hate himself for it. One day, he witnessed the Vak'auga selling an eleven-year-old girl as a slave, and also found out that his wife was pregnant. Realizing that it could have been his child had he not worked for the Vak'auga, he decided the had enough even after his wife miscarried. Leiknir was however unable to get out, since such a move would undoubtedly mean Magnus coming after him.
Magnus' ambitions grew over time, and he eventually started desiring to take over Strand. The opportunity to make it a reality finally presented itself more than a year before the Third Great War, as the Aetla Hilmir led by Nikolas approached him, offering to smuggle iron ore for them in exchange for money and alliance in taking over the throne of Strand. Magnus however first wanted them to prove themselves and after Leiknir found out that Ragnar, the son of the Governor of Strand, was going to tour Karlshus, sent them to assassinate him. Leiknir was also sent to observe them despite his protests.
The Aetla Hilmir failed to assassinate Ragnar despite slaying a number of the Governor's Guardsmen due to the intervention of Eirik, who led the soldiers protecting the Governor-Prince, and he saw Leiknir among the assailants as they fled after their failed attempt. Leiknir was later tried but acquitted as people claimed he was in Strand the entire time. He then reported about the raid to Magnus, who became convinced that the Hilmir were completely dedicated to their intent. The Vak'auga soon allied with the Mársmidr, who had them smuggle in varl skeletons and agreed to help them and the Hilmir take over Strand. This alliance left Leiknir even more worried, as he feared what Magnus would do once the three gangs took over Strand.
About eight months before the Third Great War, the Vak'auga raided a village near a river and brought the captured slaves from there to Strand. Magnus gave one of them named Runa as a gift to Leiknir, but he did not desire to keep a slave and freed her after reaching home. Runa immediately fled but found herself lost in Strand, and soon agreed to serve him. When she stated that she wanted to kill Magnus, he tested whether she had any skills required for killing, and honed her in spearing after finding out she was good at it. He also helped her learn how to navigate Strand's underworld and she became his bodyguard and messenger. Runa also became infatuated with him in time, though he always stayed loyal to his wife.
Leiknir went away with Runa from Strand for a while, but returned about two months before the Third Great War. Eirik and Valgard spotted him entering the city, but were forced to let him go as they had nothing on him. Eventually, Magnus contacted Leiknir after the Mársmidr leader Kjallak failed to capture the varls Olaf and Ullfvalgr, and told him to help with their plans. He also told him that he and Kjallak had given permission to the Aetla Hilmir leader Nikolas to raid the Great Hall, in order to capture Olaf and Ulfvalgr. Leiknir decided he had enough, telling Runa to go save the triumvirate's targets.
Runa was too late, but Eirik was able to save Olaf and Ulfvalgr along with Stefnir. Leiknir then had Runa tell Eirik to meet him at the Brenna Training Hall in order to bring down the gangs. After Eirik arrived, Leiknir told him that he knew of his escape from the Great Hall, and revealed the man leading the attackers was their leader Nikolas. He then told Eirik he had enough of his life of crime and that the triumvirate of the Aetla Hilmir, the Mársmidr and the Vak'auga was planning something major that would change Strand for the worse, though he did not know the exact details. Finally, he gave him directions to a warehouse at the western docks which would help expose them, in return for protection and immunity.
After Eirik demanded surety from Leiknir, he sent Runa along as collateral. His information turned out to be fruitful, as Eirik and his allies found a varl skull at the warehouse they had been directed to. Eirik then organized a tournament to lure one of the lieutenants of the triumvirate by getting Olaf and Ulfvalgr out in the open, and had Leiknir play his part as the Vak'auga spymaster, but told him to only inform Magnus about it very late, since it would cause the Vak'auga and the Mársmidr to struggle in finding the necessary men. Leiknir later informed the Governor's Steward Olvir that the Vak'auga had smuggled in the varl warlord Vilmundr along with some other varls into Strand, some of whom would be joining the tournament as fighters.
The Vak'auga soon became aware of Leiknir's treachery and burnt down the Brenna Training Hall, forcing him to go into hiding, with even Runa unable to find him. He had messages delivered through other spies when she tried to find him to tell him about the tournament. Eirik and his allies soon succeeded in capturing the Mársmidr lieutenant Ottar, but their plan nearly went sideways due to the City Watch of Strand, which was supposed to prevent any surprise attacks, arriving too late. Leiknir tried finding who was responsible for it, but failed to discover anything, though he did inform Runa that capturing Magnus and Kjallak was of the utmost importance.
Eirik's allies soon gained information from Ottar about the Vak'auga bringing in a shipment for the Mársmidr at the Ninth Pier, which they learnt would be their best bet to capture Magnus or Kjallak, but went without Eirik as they did not trust the City Watch, whom he wanted to get involved. Their raid however failed, with Magnus actually being revealed to be the disguise of the twins of that same name, while Stefnir and Runa were captured soon afterwards. Leiknir himself was captured as well, and taken by the Vak'auga member Fiske to the same warehouse where the two were being kept.
Knowing that Runa loved Leiknir, the Magnus twins decided to torture him first until she revealed where the second half of the Gift of Hadrborg Kjallak needed was. Skoegir however soon realized that it was on Stefnir the entire time, and the triumvirate decided to dispose of Leiknir. The Magnus twins executed him for his betrayal in front of Stefnir and Runa. The two however escaped and Runa took Eirik and his allies to the warehouse where they had been kept. Eirik regretted always hating him after finding his corpse there. They however succeeded in taking down the triumvirate after following the clues Stefnir left them, and Leknir was given a honorable funeral by the new Steward Eirik two weeks later.
It does not seem that Leiknir ever knew that there were two Magnuses, as he never expresses any awareness about it and such an important detail would have been important for Eirik and his allies to prevent any surprises while trying to take him down.