Keeper of Secrets 
Creature — Demon
$10.72 #3243 0.50 3.9 years
First strike, haste
Symphony of Pain — Whenever you cast a spell from anywhere other than your hand, this creature deals damage equal to that spell's mana value to target opponent.
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Cards similar to Keeper of Secrets 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.
Nico Minoru, Runaway81%
$16.89↑#13098↓0.00↓0.2 years↓
"Whenever you cast a spell from anywhere other than your hand, this card deals 2 damage to each opponent."
Shadow of the Goblin80%
$0.73↓#4319↓0.23↓1.0 years↓
"Whenever you play a land or cast a spell from anywhere other than your hand, this enchantment deals 1 damage to each opponent."
Unstable Amulet78%
$0.26↓#4195↓0.10↓2.2 years↓
"Whenever you cast a spell from anywhere other than your hand, this artifact deals 1 damage to each opponent."
Flaming Tyrannosaurus76%
$8.50↓#31710.462.9 years↓
"Whenever you cast a spell from anywhere other than your hand, this creature deals 3 damage to any target. Then put a +1/+1 counter on this creature."
Screamer-Killer76%
$0.54↓#9665↓0.27↓3.9 years
"Whenever you cast a creature spell with mana value 5 or greater, this creature deals 5 damage to any target."
Passionate Archaeologist75%
$16.12↑#1862↑0.434.2 years
"Commander creatures you own have "Whenever you cast a spell from exile, this creature deals damage equal to that spell's mana value to target opponent.""
Manaplasm74%
$0.18↓#16329↓0.4017.9 years↑
"Whenever you cast a spell, this creature gets +X/+X until end of turn, where X is that spell's mana value."
Memory Worm73%
$0.28↓#8445↓0.502.9 years↓
"Whenever you cast a spell from anywhere other than your hand, this creature deals 2 damage to target player. That player discards a card, then draws a card. Put a +1/+1 counter on this creature."
Vega, the Watcher72%
$0.28↓#27270.15↓5.6 years↑
"Whenever you cast a spell from anywhere other than your hand, draw a card."
Iraxxa, Empress of Mars72%
$1.29↓#6221↓0.11↓2.9 years↓
"Whenever you cast a spell from anywhere other than your hand, create a 2/2 red Alien Warrior creature token."
Enraged Flamecaster70%
$0.16↓#14671↓0.00↓0.6 years↓
"Whenever you cast a spell with mana value 4 or greater, this creature deals 2 damage to each opponent."
Vial Smasher the Fierce70%
$3.04↓#5130↓0.519.8 years↑
"Whenever you cast your first spell each turn, choose an opponent at random. Vial Smasher deals damage equal to that spell's mana value to that player or a planeswalker that player controls."
The Thirteenth Doctor70%
$0.22↓#5057↓0.422.9 years↓
"Whenever you cast a spell from anywhere other than your hand, put a +1/+1 counter on target creature."
Kellan, the Kid70%
$1.60↓#5498↓0.69↑2.4 years↓
"Whenever you cast a spell from anywhere other than your hand, you may cast a permanent spell with equal or lesser mana value from your hand without paying its mana cost. If you don't, you may put a land card from your hand onto the battlefield."
Rage Extractor70%
$0.15↓#30642↓0.87↑15.3 years↑
"Whenever you cast a spell with 
in its mana cost, this artifact deals damage equal to that spell's mana value to any target."
in its mana cost, this artifact deals damage equal to that spell's mana value to any target."
Imminent Doom70%
$0.15↓#25092↓0.31↓9.1 years↑
"Whenever you cast a spell with mana value equal to the number of doom counters on this enchantment, this enchantment deals that much damage to any target. Then put a doom counter on this enchantment."