|“||The only thing you're gonna have handed to ya at SummerSlam is your ass||„|
|~ Finn Balor to Seth Rollins prior to their match at SummerSlam|
His debut match took place during the 23 October tapings, when he and Itami defeated Justin Gabriel and Tyson Kidd. After weeks of feuding with The Ascension, Bálor and Itami defeated them at NXT TakeOver: R Evolution on 11 December. Bálor also debuted his signature body paint that day. Bálor then entered a NXT Championship number one contender's tournament, defeating Curtis Axel in the first round , Hideo Itami in the semi–finals on , and Adrian Neville in the finals on at NXT TakeOver: Rival. Bálor received his title match against Kevin Owens on 25 March, but he was unsuccessful in winning the championship after injuring his knee.
After defeating Tyler Breeze at NXT TakeOver: Unstoppable,Bálor received another title shot against Owens, which took place at The Beast in the East event, in Tokyo, Japan, where Bálor defeated kevin Owens to become the NXT Championship.
In his first title defense, Bálor defeated Kevin Owens in a ladder match at NXT TakeOver: Brooklyn on, Bálor later entered the Dusty Rhodes Tag Team Classic tournament , where he was paired with Samoa Joe, and the two defeated The Lucha Dragons (Sin Cara and Kalisto) in the first round, SmackDown in a dark match, Enzo Amore and Colin Cassady in the quarter-finals on , Dash Wilder and Scott Dawson in the semi-finals and Baron Corbin and Rhyno in the finals, on 7 October, at NXT TakeOver: Respect, to win the tournament.
After his title match with Apollo Crews, Joe turned on Bálor and attacked him, which began a feud between the two, throughout November and December, and a title match, at NXT TakeOver: London, where Bálor defeated Samoa Joe to retain.At NXT TakeOver: Dallas, Bálor successfully defended his title in a rematch aganist Joe. On 17 April, Bálor became the longest reigning NXT Champion in history . On 21 April, Bálor lost the NXT Championship to Joe at a non-televised live event , ending his reign at 292 days. On 8 June, Bálor was defeated by Joe in the first ever NXT steel cage match in a rematch for the NXT Championship at NXT TakeOver: The End, in what was his first TakeOver PPV loss, and subsequently his first loss under his "Demon" persona. Bálor wrestled his final NXT match on 30 July, when he teamed with Shinsuke Nakamura to defeat Bobby Roode and Samoa Joe in a tag team match.
On 19 July, Bálor was drafted to the Raw brand as the fifth overall pick in the returning WWE brand split , he made his first appearance for the brand on the 25 July episode of Raw , in which he won the right to compete for the newly created WWE Universal Championship at SummerSlam by defeating Rusev, Cesaro, and Kevin Owens in a fatal four-way match and then defeating Roman Reigns.