The Arrancar Invasion was a major conflict detailing the first time Soul Society had ever fought complete Arrancar. It was an invasion by Ulquiorra Cifer, Yammy Llargo, Grimmjow Jaegerjaquez and his Fracción, Luppi Antenor & Wonderweiss Margela at various intervals.
The Invasion started when Sōsuke Aizen sends Ulquiorra and Yammy to gather intel on the power level of Ichigo Kurosaki. What started out as a simple reconnaissance mission became a chaotic battle as Grimmjow and his subordinates attacked Karakura Town without permission. The result allowed the Shinigami Task Force stationed there and Ichigo to get first hand experience of fighting the new enemy. It also provided the need for Ichigo to learn to conquer his inner Hollow.
The Invasion ended with the defeats of several Arrancar at the hands of Ichigo and the Shinigami Task Force with a little extra help. But when Orihime Inoue is accused of turning traitor and going over to the Arrancar's side, Ichigo, who determined that she was taken against her will, seeks aid from Kisuke Urahara, who sends him to even greater danger in Hueco Mundo, the home of the Hollows.