幽体の兵長、ベル・ボルカ

Bell Borca notes mana values only if it's on the battlefield as a card is put into exile.

If a card in exile has {X} in its mana cost, X is considered to be 0.

If you don't play the exiled card, it remains exiled.

Look at the card in exile, not its previous zone, to determine its mana value.

While not on the battlefield, and before any cards are noted for it each turn, Bell Borca's power is 0.

You must follow the normal timing permissions and restrictions for the card exiled by Bell Borca's last ability. If it's a land, you can't play it unless you have land plays available.

You'll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.