The new episode of this show summed in to one word sad. We figure out some background information on how the system Cardinal(which runs everything in SAO works). A woman asks for the help pf Kirito and Asuna in order to save her friend Thinker from a dungeon below The Town Of Beginnings.
After fighting through some hordes of AI, they find Thinker only to find a trap. There a what seemed like Grim Reaper lied. Kirito who was not able to see it's name concluded that the creature was as strong in the level 90's. In one single blow Kirito and Asuna's HP bar was cut in half. When it looked like all hope was lost for both of them Yui comes in front of the Grim Reaper. The creature tried to attack Yui but was blocked by an invisible force shield. Then Yui was floating in the air and drew fire which ended up being a long, flaming sword. Yui then destroys the Grim Reaper in one blow.
Later Yui reveals that she remembers all her memories stating that she was an AI created by Cardinal to watch the emotions of the players. For some odd reason Cardinal did not let Yui inter react with the players when the game first launched two years ago, forcing her to observe and watch the sadness of the other players. She noticed that Kirito and Asuna who were the only players who had joy in their hearts, thus she went to go visit them, breaking the rules of Cardinal. After she reveals the information she states that she killed the Grim Reaper by deleting it in a special room (which was where she was telling Kirito and Asuna this). She said Cardinal will soon find out who deleted the program and soon delete Yui.
In the end Yui slowly starts fading away while Asuna who was hugging her was crying. She then fully disappears and Kirito states the he created her into an object, which looked like a tear that contained Yui's heart. he states that they will one day find Yui's data and retrieve her.