The Colossal Dreadmaw

You cast the copy of Emrakul, the Promised End as The Heron Moon’s last ability resolves. You do so as part of the resolution of the ability. You can’t wait to cast it later in the turn. Timing restrictions based on the card’s types are ignored.

If you choose to cast a card as Colossal Dreadmaw, while on the stack, treat that spell as though it were Colossal Dreadmaw (including the mana cost to cast it). After it resolves, treat the permanent it becomes as though it were Colossal Dreadmaw for as long as it remains on the battlefield. Once it leaves the battlefield, the card goes back to its normal self.

If you cast a spell “without paying its mana cost,” you can’t choose to cast it for any alternative costs. You can, however, pay additional costs. If the spell has any mandatory additional costs, those must be paid to cast the spell.