It's not a fight with Sephiroth in Kingdom Hearts III, but it's pretty close. The Dark-Winged Warrior event for Kingdom Hearts Union χ began on September 15th 2018. The story brings the Player and Chirithy to Olympus Coliseum, where a new tournament is being held by Hades in order to lure out Hercules, but a more villainous foe emerges from the shadows.
Will Sephiroth bring an end to the Player's life in Olympus, or will the Player show a feat of strength and fight back against the Heartless Angel? Watch to find out!