Ancient Cellarspawn
Each spell you cast that's a Demon, Horror, or Nightmare costs {1} less to cast. Whenever you cast a spell, if the amount of mana spent to cast it was less than its mana value, target opponent loses life equal to the difference.
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Official rulings (4) · Scryfall
If the spell is no longer on the stack when Ancient Cellarspawn's last ability resolves, use its mana value as it last existed on the stack.
Ancient Cellarspawn's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
For spells with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.