Eldiw · cave
Aurora Den
Aurora Den is a cave area connected to Route 14 and, after completing Battle Pass 5 ("The Song of Memory"), a gateway to the "Xenoverse" dimension itself. It offers a Shulong encounter for players who did not pick Shulong as their starter.
How to Reach It
Connected south to Route 14 and, after Pass 5, north to the Xenoverse dimension gateway.
Story Relevance
The gateway to the "Xenoverse" dimension (the title dimension of the game) opens here after completing Battle Pass 5.
Source: pokemon-xenoverse.fandom.com (Aurora Den article, raw wikitext), high confidence.