Cavern Whisperer 

Creature — Nightmare
$0.15 #10139 0.27 6.5 years
Mutate 
(If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Menace (This creature can't be blocked except by two or more creatures.)
Whenever this creature mutates, each opponent discards a card.
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Cards similar to Cavern Whisperer 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.
Silent Specter86%
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"Whenever this creature deals combat damage to a player, that player discards two cards."
+1 more matching line
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$5.12↑#22695↓0.39↑27.5 years↑
"
: Target opponent discards two cards. Activate only during your turn, before attackers are declared."
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$0.15#28064↓0.3324.0 years↑
"Whenever this creature deals combat damage to a player, you may sacrifice it. If you do, that player discards three cards."
+1 more matching line
Headhunter81%
$0.30↑#26514↓0.2824.0 years↑
"Whenever this creature deals combat damage to a player, that player discards a card."
+1 more matching line
Liliana's Specter80%
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"When this creature enters, each opponent discards a card."
Scepter of Fugue80%
$0.33↑#26309↓0.17↓17.7 years↑
"
,
: Target player discards a card. Activate only during your turn."
Caligo Skin-Witch80%
$0.11↓#24620↓0.65↑8.5 years↑
"When this creature enters, if it was kicked, each opponent discards two cards."
Rafter Demon80%
$0.08↓#26004↓0.00↓7.7 years↑
"When this creature enters, if its spectacle cost was paid, each opponent discards a card."