Riverchurn Monument

{1}, {T}: Any number of target players each mill two cards. (Each of them puts the top two cards of their library into their graveyard.)
Exhaust — {2}{U}{U}, {T}: Any number of target players each mill cards equal to the number of cards in their graveyard. (Activate each exhaust ability only once.)

Aetherdrift

{1}, {T}: Any number of target players each mill two cards.
Exhaust — {2}{U}{U}, {T}: Any number of target players each mill cards equal to the number of cards in their graveyard.

Aetherdrift Promos

Exhaust abilities can be activated any time you could normally activate an ability.

If an ability triggers whenever you activate an exhaust ability, that ability resolves before the exhaust ability resolves.

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.