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 in order for its ability to trigger.

Casting a spell for an alternative cost, such as a bestow cost, doesn’t change its converted mana cost. Similarly, casting a spell with an additional cost, such as a spell with a strive ability, doesn’t change its converted mana cost. For example, casting Gnarled Scarhide (a card with mana cost and bestow cost ) for its bestow cost will cause Eidolon of the Great Revel’s ability to trigger.