Riverchurn Monument is a rare Artifact card from Aetherdrift Promos (PDFT), collector number 57p. Current market price: $2.38 (foil $2.63), TCGplayer market prices via Scryfall.
{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.)