Eidolon of the Great Revel

• WWizards of the Coast2018-03-16
  Official Ruling
  Casting Eidolon of the Great Revel doesn't cause its own ability to trigger. It must be on the battlefield when the spell is cast for its ability to trigger.
• WWizards of the Coast2018-03-16
  Official Ruling
  The amount of mana spent to cast a spell doesn't change its mana value. For example, a card cast with its morph ability has a mana value of 0 even though was paid.
• WWizards of the Coast2018-03-16
  Official Ruling
  The mana value of a spell with in its mana cost includes the value chosen for X.

Rulings via Scryfall