Lee "Duke" Ho-Seong joined the KT Rolster Bullets sometime in Season 3 under the name "Leopard" but didn't play with them until the IEM Season VIII World Championship, when he became the starting top laner. The Bullets swept the whole tournament and beat Fnatic 3-0 in the final. They came back to Korea to participate in HOT6iX Champions Spring 2014 where they made it to the quarterfinals but lost 3-2 to NaJin White Shield. Shortly after, Leopard changed his name to Duke and joined NaJin Black Sword as their new top laner. He played his first tournament with the team at HOT6iX Champions Summer 2014. Sword made it to the quarterfinals but lost 3-2 to SK Telecom T1 S.
Due to the changes to the OGN rules, Sword and Shield were forced to merge, forming NaJin e-mFire. Shield's top laner, Save, departed from the organization, giving Duke sole possession of the starting position. The team was invited to SBENU Champions Spring 2015 where they placed 6th in the regular season and missed the playoffs. Despite his team's mediocre performance, Duke earned the MVP award for the season. NaJin placed fifth in the regular season of SBENU Champions Summer 2015 and qualified for the playoffs but they lost in the first round 2-1 to the KOO Tigers. They earned enough circuit points to make it to the 2015 Season Korea Regional Finals but lost 3-0 in the first round to the Jin Air Green Wings.
NaJin played in the 2015 LoL KeSPA Cup where they lost 2-1 to Rebels Anarchy in the first round. Duke left NaJin in December and joined SK Telecom. In March, SK Telecom attended IEM Katowice despite being only the sixth-place LCK team at the time. They brought substitute jungler Blank in place of bengi and swept through the tournament, winning the title after only seven games. After their international success, they returned to Korea and placed third in the regular season, earning themselves a spot in the playoffs. SKT won the playoffs, winning their third championship in a row and for the first time a seed to the 2016 Mid-Season Invitational.
Their first place finish in the spring season earned them a spot in the 2016 Mid-Season Invitational. Notably, SK Telecom T1 were the only team to return to 2016 MSI from 2015 MSI. As the reigning domestic champions of Korea, SK Telecom T1 came into the tournament as massive favorites to win. However, their group stage performances were very lackluster as they dropped games to China's Royal Never Give Up, NA's Counter Logic Gaming, and two games to the LMS Flash Wolves. They ended their group stage with a 6-4 record, seeding them into the bracket stage at fourth place. Matched up against Royal Never Give Up in the semifinals, SK Telecom started to regain their form, beating them 3-1. They then proceeded to beat Counter Logic Gaming 3-0 to win the event and give them the trophy that barely escaped them last year.
During the 2016 LCK Summer Split, SK Telecom started the season strong with a 5-0 score in series and 10-0 game score. However, they started dropping sets to lower ranked teams, notably the Afreeca Freecs, who they failed to win even a single game against the entire season. With a 13-5 scoreline in series, SKT placed second in the round robin, beating KT Rolster via game score tiebreaker. However, during the 2016 LCK Summer Playoffs, they met KT Rolster in the semifinals, where they were reverse swept 3-2 after going up 2-0 in the first two games. This gave them third place for the summer split. Combined with their first place spring finish, as well as the victory of the ROX Tigers over KT Rolster, SKT qualified to the 2016 Season World Championship on points as Korea's second seed.
At the 2016 Season World Championship, SK Telecom T1 were placed into Group B, along with LMS's Flash Wolves, NA's Cloud 9, and China's I May.