As Foretold (WHO)

Enchantment

At the beginning of your upkeep, put a time counter on this enchantment. Once each turn, you may pay 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 this enchantment.

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