Hellcarver Demon

If you cast a card “without paying its mana cost,” you can’t pay any alternative costs. On the other hand, if the card has optional additional costs (such as kicker or multikicker), you may pay those when you cast the card. If the card has mandatory additional costs (such as Momentous Fall does), you must pay those when you cast the card.

Any cards you can’t cast (because they’re lands or they have no legal targets, perhaps) or choose not to cast simply remain exiled.

You cast nonland cards from exile as part of the resolution of Hellcarver Demon’s ability. You may cast those cards in any order. Timing restrictions based on the card’s type (such as creature or sorcery) are ignored. Other restrictions are not (such as “Cast [this card] only before the combat damage step”). You cast all of the cards you like, putting them onto the stack, then Hellcarver Demon’s ability finishes resolving. The spells you cast this way will then resolve as normal, one at a time, in the opposite order that they were put on the stack.