in its mana cost, X is 0 when determining its mana value.Chthonian Nightmare exchanges a creature on the battlefield for one in the graveyard while using energy to set the size of the return. Its entry supplies three energy. Activating it spends energy equal to the target creature's mana value, sacrifices a creature, and returns the enchantment to your hand before the chosen creature comes back.
That return-to-hand cost lets you cast the enchantment again for another entry and another three energy. Cheap creatures with useful entry or death abilities make the exchanges productive. A small creature that already drew a card can become a removal creature, then later exchanges can continue rebuilding the board according to what the graveyard contains.
The target must already be in the graveyard when you activate. You cannot choose the creature you are about to sacrifice as the target of that same activation because target selection happens before paying the sacrifice cost. You also need the required energy available when paying; the enchantment's next cast cannot retroactively fund the current return.
Returning a creature with mana value two spends two of the three energy from the first entry. If you later recast the Nightmare, its new entry adds three more energy to the remaining one. Keeping track of that reserve can enable a larger return later in the sequence.
Sorcery-speed activation means this is a planned engine for your own main phase rather than an instant-speed rescue. Opponents can disrupt the graveyard target after costs are paid, even though the Nightmare itself has already returned to hand. Black creature-value decks can use the card for repeated exchanges, especially when their low-cost creatures do useful work each time they enter or die. Count energy and mana separately when planning several activations in a turn.
