Riverchurn Monument 

Artifact
$1.32 #3468 0.41 1.7 years
Exhaust — 

,
: Any number of target players each mill cards equal to the number of cards in their graveyard. (Activate each exhaust ability only once.)
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Cards similar to Riverchurn Monument 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.
Keening Stone86%
$0.34↓#9604↓0.94↑16.5 years↑
"
,
: Target player mills X cards, where X is the number of cards in that player's graveyard."
Jace, Memory Adept78%
$1.18↓#5820↓0.56↑15.3 years↑
"0: Target player mills ten cards."
+1 more matching line
Grindclock76%
$0.33↓#14254↓0.16↓16.1 years↑
"
: Target player mills X cards, where X is the number of charge counters on this artifact."
Seer of the Last Tomorrow73%
$0.03↓#27825↓0.29↓9.3 years↑
"
,
, Discard a card: Target player mills three cards."
Stern Mentor72%
$0.05↓#28527↓0.06↓14.5 years↑
"As long as this creature is paired with another creature, each of those creatures has "
: Target player mills two cards.""