If you meant the 50th episode of the second series, the tone is much darker as the Barian Invasion reaches its peak.
However, "Zexal Second" (or Zexal II ) typically refers to the second half of the series, starting from Episode 74. If you are looking for the 50th episode specifically within the Zexal II sequel (which would be Episode 123 of the overall series), that episode is titled (Japanese title: "Hero's Arrival! The One Standing Against the Three Emperors!"). Yu-Gi-Oh! ZEXAL — Episode 50: "Party Panic"
The celebration is interrupted when a group of "duelists" led by a man named Nelson Andrews (who plays the character "ESPer Robin") causes a scene. Yuma eventually faces off against a mysterious challenger while the tension for the upcoming finals builds. Yu-Gi-Oh! Zexal Second Episode 50
This episode takes place on the eve of the World Duel Carnival (WDC) finals in Heartland City.
In the original series numbering, of Yu-Gi-Oh! Zexal is titled "Party Panic" (Japanese title: "The Grand Feast of Justice! Calling on ESPer Robin!!"). If you meant the 50th episode of the
Yuma, Astral, Tori, Shark, Kite, and the introduction/reappearance of various WDC finalists.
The Barian Emperors—Girag, Alito, Mizar, Dumon, Marin, and Nash—have launched a full-scale assault on Earth. Yuma is incapacitated after a psychological clash with Nash (Shark), leaving his friends to defend the city. The One Standing Against the Three Emperors
This episode marks the beginning of the final series of duels where Yuma's friends (like Anna, Nistro, and Nelson) sacrifice themselves to buy time against the overwhelming power of the Barians.