Outside Eldiw · island
Shinobi Island
Shinobi Island is a standalone area outside Eldiw, unlocked after receiving the first Battle Pass ("Furious Battles") at a Pokémon Center, following defeat of the Champion atop Mt. Zodiac. It hosts a Furious Battle boss fight against a Fire/Dark Greninja X.
How to Reach It
Unlocked via the first Battle Pass, obtainable at a Pokémon Center after beating the Champion on Mt. Zodiac.
Story Relevance
Post-Champion content unlocked by the first Battle Pass ("Furious Battles"), featuring a superboss fight against a Fire/Dark Greninja X.
Source: pokemon-xenoverse.fandom.com (Shinobi Island article, raw wikitext), high confidence.