Killian Dain

Mackle trained in Scotland at NWA Scotland under Robbie Brookside (now a trainer at WWE NXT). Of Brookside, he stated; "I don't think there is enough hyperbole to explain how good a coach he is...I can't think of a better coach I'd met in any sport".[15] After training at NWA Scotland, Mackle debuted for the Scottish Wrestling Alliance at the age of 20 under the ring name Damian O'Connor. He originally wrestled in Scotland and the North of England as a tag team called "Britain's Most Wanted" with Scott Renwick. Together the two won a variety of tournaments and tag team championships in promotions such as SWA (4 times), W3L (2 times), SSW and 3CW. While they never officially split up, they would both focus on singles competition around 2009. This proved to be fruitful for both and O'Connor started touring his native Ireland, the United Kingdom, mainland Europe and the United States. O'Connor would win the Heavyweight Titles in SWE, W3L, XWA, Pride as well as the Laird of the Ring Title in SWA. In 2009, O'Connor took over the SWA training school named the "Source Wrestling School". O'Connor has trained numerous wrestlers, including Joe Coffey, Joe Hendry and Nikki Cross.[17]

O'Connor wrestled sporadically for ICW between 2009 and 2012 under various gimmicks, but his return in November 2013 saw him debut as Big Damo. Damo defeated Chris Renfrew for the ICW World Heavyweight Championship in Belfast on 27 February 2016,[18] marking the first time the title changed hands outside of Glasgow and making Damo the first ever Irish-born champion. On 31 July 2016, at Shug's Hoose Party III, Damo was defeated for the Championship by Joe Coffey.[19]

On 11 November 2018. O'Connor (as Killian Dain) returned to ICW for one night only, defeating Mikey Whiplash.[20]

In December 2014, Damo unsuccessfully challenged Drew Galloway for the EVOLVE Championship in Inverness, Scotland.[21]

In 2014, Damo debuted in Revolution Pro Wrestling.[22] In 2015, he wrestled NJPW greats Tomohiro Ishii,[23] Hiroshi Tanahashi[24] Shinsuke Nakamura,[25] as well as Tommaso Ciampa.[26] In 2016, he wrestled international wrestlers "Speedball" Mike Bailey,[27] Roderick Strong,[28] Dalton Castle,[29] Big Daddy Walter[30] and Matt Sydal.[31]

In October 2015, Damo debuted for Global Force Wrestling (GFW) on their UK Invasion tour, losing to Bram.[32]

In January 2016, Damo appeared for Total Nonstop Action (TNA) as part of their Maximum Impact tour.[33]

On the 1 March episode of IMPACT Wrestling, Big Damo unsuccessfully challenged Eric Young for the TNA King of the Mountain Championship.[34] At TNA One Night Only: Joker's Wild, Big Damo and Jimmy Havoc were defeated by Drew McIntyre and Mike Bennett.[35] On the 8 March episode of IMPACT Wrestling, Big Damo competed in a King of the Mountain match for Young's King of the Mountain Championship also featuring Havoc, Ospreay and Bram; Young successfully retained.[36]

In June 2016, Mackle signed with WWE to perform on their NXT brand.[37][38] On 19 October 2016, WWE announced that Mackle had signed a developmental contract with the company and had arrived at the WWE Performance Center in Orlando, Florida.[13] On 4 November, Mackle appeared at an NXT live event and cut a promo.[39] Mackle made his in-ring debut at an NXT live event on 11 November, defeating Rich Swann under the ring name Damian O'Connor.[40] He then reverted to using the ring name Damo.

On the 7 December episode of NXT, Damo attacked No Way Jose and seemed to align himself with the heel stable SAnitY.[41] In January 2017, he began to wrestle under the new ring name Killian Dain.[42] On the 18 January episode of NXT, Dain joined SAnitY in an official capacity after attacking Tye Dillinger and accepting Sawyer Fulton's jacket from Eric Young.[43] On the 25 January episode of NXT, Dain appeared as part of SAnitY for the first time, attacking Chris Atkins by Young's order.[44] On the 8 February episode of NXT, Dain teamed with Young and Alexander Wolfe to defeat Dillinger, No Way Jose and Roderick Strong.[45] Earlier in the night, Dain and Wolfe defeated The Bollywood Boyz.[46] At WrestleMania 33, Dain made it to the final three in the André the Giant Memorial Battle Royal before being eliminated by eventual winner Mojo Rawley.[47] On the 19 July episode of NXT, Dain lost to Drew McIntyre in an NXT Championship No. 1 Contender's match, marking his first televised loss.[48]

On the 28 March 2018 episode of NXT, Dain was named as one of six participants in ladder match at NXT TakeOver: New Orleans to determine the inaugural NXT North American Champion.[49] He, however was unsuccessful in winning.[50] On the 18 April episode of NXT, Dain lost a No Disqualification match to Lars Sullivan.[51]

During the 2018 WWE Superstar Shake-up, SAnitY (excluding Cross) were drafted to SmackDown.[52] The group debuted on the 19 June episode of SmackDown LIVE where they attacked The Usos.[53] The following week, SAnitY lost to The Usos and Jeff Hardy.[54] On the 3 July episode of SmackDown LIVE, the group attacked The New Day.[55] This led to a tables match between the two teams at Extreme Rules, which SAnitY won.[56] On the 24 July episode of SmackDown LIVE, SAnitY and The New Day faced off in the first round of the WWE SmackDown Tag Team Championship No. 1 Contender's tournament. SAnitY (represented by Dain and Wolfe) were defeated.[57]

The group competed in only two more televised matches throughout the remainder of 2018, while primarily competing at live events.[58]

After a near four-month hiatus from television, SAnitY returned on the 2 April 2019 episode of SmackDown LIVE, losing a three-on-one Falls Count Anywhere match to The Miz.[59]

During the 2019 WWE Superstar Shake-up, Young was drafted to Raw.[60] With Wolfe joining NXT UK,[61] the group was officially disbanded.

Dain made his return to NXT live events on 16 May, defeating Jermaine Haley.[62] On the 3 July episode of NXT, a promo advertising Dain's return was aired.[63] He made his return on the 17 July episode of NXT, attacking Matt Riddle following Riddle's match against Arturo Ruas.[64] This led to a match between the two being scheduled for the 7 August episode of NXT. However, Dain attacked Riddle before the match could begin.[65]