Keeper of Secrets 
Creature — Demon
$7.59 #3300 0.36 4.0 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%
$17.45↑#11599↓0.400.3 years↓
"Whenever you cast a spell from anywhere other than your hand, this card deals 2 damage to each opponent."
Shadow of the Goblin80%
$0.89↓#4110↓0.19↓1.1 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%
$0.21↓#4273↓0.17↓2.3 years↓
"Whenever you cast a spell from anywhere other than your hand, this artifact deals 1 damage to each opponent."
Flaming Tyrannosaurus76%
$8.43↑#31790.363.0 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%
$0.58↓#9885↓0.65↑4.0 years
"Whenever you cast a creature spell with mana value 5 or greater, this creature deals 5 damage to any target."
Passionate Archaeologist75%
$13.90↑#1875↑0.48↑4.4 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%
$0.20↓#16463↓0.00↓18.0 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%
$0.40↓#8580↓0.373.0 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%
$0.30↓#2573↑0.11↓5.7 years↑
"Whenever you cast a spell from anywhere other than your hand, draw a card."
Iraxxa, Empress of Mars72%
$1.23↓#6317↓0.09↓3.0 years↓
"Whenever you cast a spell from anywhere other than your hand, create a 2/2 red Alien Warrior creature token."
Enraged Flamecaster70%
$0.14↓#14469↓0.12↓0.7 years↓
"Whenever you cast a spell with mana value 4 or greater, this creature deals 2 damage to each opponent."
Kellan, the Kid70%
$1.62↓#5213↓0.07↓2.5 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."
Vial Smasher the Fierce70%
$3.00↓#5425↓0.319.9 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."
Imminent Doom70%
$0.13↓#25326↓0.21↓9.3 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."