Hunter x Hunter: The Last Mission

The film begins before the plot of Hunter x Hunter, some decades ago, when the chairman of the Hunters Association, Isaac Netero, defeats a powerful enemy known as Jed. Back to the present, a group of cloaked men storm into a prison and free the captives there, recruiting them for their plan. Some time later, Gon and Killua take a break from his expeditions with Kite to pay another visit at Heaven's Arena and cheer for their friend Zushi in the Battle Olympia Tournament with Wing and Biscuit. Meanwhile, Leorio, who is also bound to meet them, is attacked and knocked out by the cloaked men. Before the tournament begins, Gon and Killua pay a visit to Netero, who is also at the building, but the Arena is taken over by the mysterious men, with one of them, Gaki, replacing Zushi's first opponent and defeating him, another one, Shura, taking over the security system and a third one, Rengoku, stabbing herself and putting a curse on Netero to seal his Nen and immobilizing him. The ringleader then appears and knocks Gon and Killua down from the tower, but the duo manage to avoid the fall and return to the building, several floors below. Read more on Wikipedia.