After the chaos caused by the Nine Anonymous Stars, the Siren Hymn Temple was filled with gloom and in a state of emergency. Even the three empires had noticed this, but they didn't do anything and silently observed from the shadows.
Furthermore, they were busy tracking down the Legendary Key's location given to them by the Ocean Oracle, and since this was an internal matter of the Siren Hymn Temple, they also didn't want others to interfere in this.
However, because the main temple sent huge forces to investigate and clean up their rotten priest lords, this decreased the security of the main temple by at least half.
Which made it easier for those who knew their way around this place to sneak in. However, it was still quite difficult with all the defensive formations in this place.