From the introduction:
Long ago, Kelizandri, the evil elemental lord of water, conspired with the villainous lords of air, earth, and fire to trap the four good elemental lords within arcane gems. They imprisoned Ranginori, the good elemental lord of air, in a gem that came to be called the Untouchable Opal. Thought lost for eons, the artifact recently found its way into the hands of the Pathfinder Society. With the prisoner of the stone identified, the Society now seeks to free the demigod to win his favor and gain easier access to the Plane of Air.
For tier 1 - 5.