Ep 21. The Truth of Alfheim

In Sword Art Online season 1 episode 21, titled "The Truth of Alfheim", the adventure continues as Kirito and Leafa make their way to World Tree. As they fly through the skies, Kirito's words of encouragement give Leafa a new sense of determination.

Once they arrive at World Tree, they meet up with Recon and begin to climb towards the top. Along the way, they encounter a powerful creature that almost kills them, but they manage to defeat it.

As they approach the top, they see the entrance to the sealed area, which is guarded by players from the top guilds. Kirito and Leafa put on disguises and infiltrate the guarded area to get a closer look.

Inside, they find a room with a large, mysterious machine. It is the creation of the game's developers, and that the whole purpose of this game was to create a new kind of AI. Kirito is shocked when he realizes that Aincrad was also an experiment, and that all of the players were part of a grand experiment to create the ultimate AI.

But as they are leaving the room, they are discovered and forced to battle with one of the top guilds. Kirito unleashes his full power and defeats the opponents with ease.

Meanwhile, Asuna is still trapped in her virtual prison, and her situation is getting worse. She is physically and mentally abused by her captor, and she starts to lose hope of ever escaping. However, she manages to find a glimmer of hope when she discovers a hidden item that could change her fate.

As the episode comes to a close, several plot threads come together. Kirito and Leafa discover that they are related in the real world, and there is a touching moment of realization between the two. Asuna also creates a chance to escape, while Kirito and Leafa make a decision to work together to save their friends.

Overall, Sword Art Online season 1 episode 21 is an action-packed episode full of surprises and revelations. It sets up the stage for the final showdown, and it will leave viewers eager to see what happens next.