The Ruins Of Enoshima Island

You must be apart of the Iramasha clan to active this island. First, you must have at least 2 Angelic or Demonic Emeralds.

Then, you must use the Emeralds to empower an ancient key that will awaken Angel Island. Upon activation the island will arise from out of the ground and appear in the sky.

This is where the Iramasha's put their Ultimate Emerald; The Master Emerald.

Once you obtain the Master Emerald you'll have supreme control over the Angelic Emeralds and Demon Emeralds.

That is....if you can get past the guardian's who live inside these ruins to protect the Master Emerald.