Ancient Cellarspawn 
Enchantment Creature — Horror
$6.16 #2452 0.35 1.9 years
Each spell you cast that's a Demon, Horror, or Nightmare costs less to cast.
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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Cards similar to Ancient Cellarspawn expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Herald of Slaanesh77%
$5.82↓#5781↓0.19↓3.9 years↑
"Demon spells you cast cost 
less to cast."
less to cast."