Mystery Key 

Artifact — Equipment
$0.09 #17116 0.00 4.2 years
When equipped creature deals combat damage to a player, sacrifice this Equipment. If you do, draw three cards.
Equip
(
: Attach to target creature you control. Equip only as a sorcery.)
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Cards similar to Mystery Key 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.
Impaler Shrike84%
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"Whenever this creature deals combat damage to a player, you may sacrifice it. If you do, draw three cards."
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$0.09#11721↑0.004.2 years
"Whenever equipped creature deals combat damage to a player, scry 1, then draw a card."
+1 more matching line
Rogue's Gloves80%
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"Whenever equipped creature deals combat damage to a player, you may draw a card."
+1 more matching line
Robe of the Archmagi76%
$0.42↑#5088↑0.15↑4.2 years
"Whenever equipped creature deals combat damage to a player, you draw that many cards."
Mighty Servant of Leuk-o76%
$0.37↑#5956↑0.35↑4.2 years
"Whenever this Vehicle becomes crewed for the first time each turn, if it was crewed by exactly two creatures, it gains "Whenever this creature deals combat damage to a player, draw two cards" until end of turn."
Acquisition Octopus74%
$0.15↑#10744↑0.044.5 years
"Whenever this creature or equipped creature deals combat damage to a player, draw a card."
Mask of Riddles74%
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"Whenever equipped creature deals combat damage to a player, you may draw a card."
+1 more matching line
Scroll Thief73%
$0.06↓#170950.36↑16.1 years↑
"Whenever this creature deals combat damage to a player, draw a card."
Nine-Tail White Fox73%
$0.21↑#22046↓0.31↑8.2 years↑
"Whenever this creature deals combat damage to a player, draw a card."