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 Nerv Gear and begin their fateful adventure in the Sword Art Online game. 
 
(Source: Anime News Network, edited)
 
Note: Coincidentally, the release date of this video lines up with the fictional date of when the servers of the Sword Art Online game launched in-universe.

Sword Art Online: FULLDIVE - Opening Eizou