"It is returning through the dark. And then, Doctor? Oh, but then he will knock four times."

As Foretold

At the beginning of your upkeep, put a time counter on As Foretold.
Once each turn, you may pay {0} rather than pay the mana cost for a spell you cast with mana value X or less, where X is the number of time counters on As Foretold.

Doctor Who

Wilds of Eldraine: Enchanting Tales

A card's mana value is determined solely by the mana symbols printed in its upper right corner. The mana value is the total amount of mana in that cost, regardless of color. For example, a card with mana cost {1}{U}{U} has mana value 3. Ignore any alternative costs, additional costs, cost increases, or cost reductions that could apply to it. A card with no mana cost has a mana value of 0.

All counters with the same name are indistinguishable from other counters with that name. Cards from the Time Spiral block that interact with time counters will interact with As Foretold.

If a spell has no mana cost, its mana value is 0. You can cast it with As Foretold's alternative cost.

If the card has X in its mana cost, you must choose 0 as the value of X when casting it for another cost that doesn't include X. For example, if there are five time counters on As Foretold and you choose to use its effect to cast Pull from Tomorrow, X must be 0. X can't be 2, even though the spell's mana value would be 4.

If you control multiple As Foretolds, you may cast one spell for each of them paying {0}.

If you're casting a split card, only the mana value of the half you're casting is compared to the number of time counters on As Foretold. This is because only that half is on the stack at the time that you choose to apply the effect of As Foretold.

If you cast a spell for an alternative cost of {0}, you can't pay any other alternative costs, such as emerge costs. You can, however, pay additional costs, such as bargain. If the spell has any mandatory additional costs, such as that of Tormenting Voice, you must pay those to cast the spell.