Hunter x Hunter (2011)

S01E33 An x Empty x Threat!

After a two-month break, Gon and Killua are finally given the green light to train in Nen once more. Soon after, they encounter Gido, Sadaso, and Riehlvelt, who challenge them to battle in the arena. Gon and Killua decline the offer, but Killua can't shake off the feeling that Sadaso is acting oddly.