Opening video for the "Sword Art Online -FULLDIVE-" event which took place at the Tokyo Garden Theater. This memorial event was to celebrate 10 years of the Sword Art Online anime, with an original story by Reki Kawahara performed by the anime's seiyuus via live voice drama and musical guest performances by artists involved with the anime's music throughout the decade.
The animation shows the main characters of the Aincrad Arc right before they put on the NerveGear and begin their fateful adventure in the Sword Art Online game.
(Source: Anime News Network, edited)