Erayo, Soratami Ascendant // Erayo's Essence

• WWizards of the Coast2005-06-01
  Official Ruling
  Each Ascendant is legendary in both its unflipped and flipped forms. This means that effects that look for legendary creature cards, such as Time of Need and Captain Sisay, can find an Ascendant.
• WWizards of the Coast2005-06-01
  Official Ruling
  If Erayo, Soratami Ascendant flips while an opponent’s first spell for the turn is still on the stack, the ability of Erayo’s Essence doesn’t trigger because the opponent’s spell has already been cast. The spell isn’t countered.
• WWizards of the Coast2005-06-01
  Official Ruling
  Erayo’s Essence triggers only for the first spell an opponent casts in a given turn. It doesn’t trigger off any other spell that player casts after the first.
• WWizards of the Coast2005-06-01
  Official Ruling
  If you have multiple opponents, Erayo’s Essence can trigger once for each opponent.

Rulings via Scryfall