Belle Cove needs the Mythics in order for the magic to balance.
Not everyone is willing to play nice.

The last time all four races openly lived with humans, there had been less than a hundred people in the region. Now there were hundreds of thousands.

The Supernatural Police need to to step up their game if they're going to keep a semblance of peace in the region and not have every single non-human suddenly exposed to the whole world.

This is Sid & Sin's legacy. It's what the prophecy spoke about.
Are they ready - and able - to answer the call?