Riverchurn Monument
Artifact
, 
: Any number of target players each mill two cards. (Each of them puts the top two cards of their library into their graveyard.)
Exhaust — 
, : Any number of target players each mill cards equal to the number of cards in their graveyard. (Activate each exhaust ability only once.)
Anthony Devine
Aetherdrift Promos (PDFT)
• #57sRulings
If an ability triggers whenever you activate an exhaust ability, that ability resolves before the exhaust ability resolves.
(wotc • 2025-02-07)
If an exhaust ability of a permanent is activated, and then that permanent leaves the battlefield and returns to the battlefield, it becomes a new object so its exhaust ability can be activated again.
(wotc • 2025-02-07)
Exhaust abilities can be activated any time you could normally activate an ability.
(wotc • 2025-02-07)
Rulings
If an ability triggers whenever you activate an exhaust ability, that ability resolves before the exhaust ability resolves.
(wotc • 2025-02-07)
If an exhaust ability of a permanent is activated, and then that permanent leaves the battlefield and returns to the battlefield, it becomes a new object so its exhaust ability can be activated again.
(wotc • 2025-02-07)
Exhaust abilities can be activated any time you could normally activate an ability.
(wotc • 2025-02-07)