Cheapest priced copy (Duskmourn: House of Horror Commander): $11.99 at Card Kingdom, near mint, their price list as of .
Ancient Cellarspawn 
Enchantment Creature — Horror · 3/3 · Rare · Duskmourn: House of Horror Commander · #16
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.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $11.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duskmourn: House of Horror Commander | 16 | Rare | $11.99 | — |
| Duskmourn: House of Horror Commander | 47 | Rare | $11.99 | — |
4 rulings from Wizards of the Coast
- 20 September 2024If 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.
- 20 September 2024Ancient Cellarspawn's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
- 20 September 2024To 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.
- 20 September 2024For spells with {X} in their mana costs, use the value chosen for X to determine the spell's mana value.