On July 22, 2021, Stray Kids released a two-and-half-minute-long official comeback trailer named "Thunderous" and announced their second studio album
Noeasy was scheduled for release on August 23. The trailer was directed by Lee Hye-sung and produced by Sushivisual; it has a
superhero theme and shows the members transforming into avenger corps against the ear-shaped "sound monster" by inventing technology and weapons, and brainstorming strategies in an apocalyptic world. On August 2, they released an exclusive clip with behind-the-scenes footage of the trailer. In the clip, the members tease the undisclosed lead single; Felix appears alone and greets the group's fans. The group released snippet videos for the eight of the twelve tracks: "Cheese", "Red Lights", "Gone Away", "Surfin'", "Domino", "The View", and "Secret Secret". The teaser images were uploaded in three sets. The first concept shows a red
avant-garde outfit with a smoky background. The second depicts a black-and-white outfit with plain white or
hanok background. The complete track list of
Noeasy was announced on August 12, 2021, with "
Thunderous" as the album's lead single, and the previous releases "Wolfgang" and "Mixtape: Oh". Two music-video teasers of"Thunderous" were released on August 20 and 21, and the performance-video teaser with SKZoo's costumes was released the following day. The online cover artwork, and the 24-minute-long documentary video
Intro: "Noeasy" introducing the album, were also released with the album on August 23. To promote the album, the group held an online listening party via Stationhead on August 27 (
KST), and appeared on several television and radio programs, including
domestic music programs,
After School Club, ''
Day6's Kiss the Radio, and Cultwo Show.'' In addition to the lead single, the accompanying music videos for B-sides "The View", "Cheese", "Red Lights", "Surfin'", and "Gone Away", and live videos for "Sorry I Love You" and "Secret Secret" were also released.
Kingdom Week Stray Kids appeared on
Kingdom Week , a television variety show that benefits the winner of
Kingdom: Legendary War, which aired on Mnet for seven episodes from August 17 to 23. The first and second episodes showed the group behind the scenes during
Kingdom: Legendary War, with commentary by Felix and Seungmin. In the third, fourth, and fifth episodes, the members go to a villa to rest and play a game called "buddy catcher". The sixth episode includes the group's best performance chosen by the group members. The final episode, airing the day as the album's release, called
Stray Kids Comeback Show "Noeasy", showcases the album.
Singles "Thunderous" is the lead single from
Noeasy. and posted to
YouTube,
Naver TV, and
V Live. Both the single and the video were released alongside the album on August 23, 2021. number three on the US
World Digital Song Sales, and number eighty on the
Billboard Global 200, and took six wins from music programs: twice on
Show Champion, twice on
M Countdown, once on
Music Bank, and once on
Inkigayo. The accompanying music video reached 50 million views on YouTube in six days of release and 100 million views in fifty-five days, making it the group's fastest and fifth 100-million-view music video.
Live performances Stray Kids gave the debut performance of
Noeasy with a showcase, called
Stray Kids Comeback Show "Noeasy", which was held on the day as the album's release as part of the final episode of
Kingdom Week . The group performed the lead single "Thunderous", and the B-side "The View" for the first time at the show, alongside the show's final competition song "Wolfgang", and "Grow Up" from their debut EP
I Am Not (2018). The group promoted "Thunderous" on South Korean music programs
M Countdown,
Music Bank,
Show! Music Core,
Inkigayo, and
Show Champion for three weeks, August 26 – September 12, along with "The View" in the first week of promotion, "Sorry, I Love You" was performed for the first time at
Unite On: Live Concert on November 6. Stray Kids also performed songs from
Noeasy at several awards ceremonies and year-end annual music shows. The group gave the debut performances of the sub-unit tracks "Red Lights", "Surfin'", and "Gone Away" as a medley alongside "Thunderous" at the 2021
The Fact Music Awards on October 2. They performed "Domino" and "Thunderous", and also showed Lee Know's solo dance performance and 3Racha's rap performance, at the 2021
Asia Artist Awards on December 2. "Cheese" was given its debut performance on December 11 at the
2021 Mnet Asian Music Awards as part of medley titled
Stray Kids World Domination, alongside the "hero" version of "Thunderous" and the unreleased song "Hey, Monster", whis was performed by 3Racha. The group performed "Thunderous" with a new arrangement at the
36th Golden Disc Awards on January 8, 2022, alongside "Top" and "Wolfgang". The group performed "Thunderous" at the 2021
KBS Song Festival on December 17 at
Namhansanseong; the performance was described as a "mega performance" and "action movie-like feeling". They performed the Christmas version of "Thunderous" at 2021
SBS Gayo Daejeon on December 25, using canes and group dance, and parodying
Squid Game ttakji, and
dalgona game, alongside "
Winter Falls" from the single album
Christmas EveL, and covered
IU's "
Merry Christmas Ahead" performed by the members Changbin, Han, Felix, Seungmin, and I.N, as part of Christmas carol medley with other artists. They closed the year with a performance of "Thunderous" at 2021
MBC Gayo Daejejeon on December 31, adding the ringing of a New Year's bell before the New Year ceremony. As an ambassador for the Korea Pavilion of
Expo 2020 in
Dubai,
United Arab Emirates, Stray Kids performed "Thunderous" as part of the opening show of the Korea Pavilion's Korea National Day with the Little Angels and K-Tigers on January 17, 2022. The group also performed "Thunderous" alongside "Miroh", "God's Menu", and "Back Door" at the Korea National Day K-Pop Concert on the same day. The debut performance of "Silent Cry" was given at the group's second fan meeting on February 12 and 13 at
Olympic Hall. ==Critical reception==