Is Riverchurn Monument legal in Standard, Modern, and Commander?

Riverchurn Monument is legal in: Standard, Modern, Pioneer, Legacy, Vintage, Commander, Historic.

Riverchurn Monument

Q: What type of card is Riverchurn Monument?

Riverchurn Monument is an Artifact. Its oracle text reads: "{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.)"

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.)