Harpies are among the most detested creatures in all of Theros... and they thoroughly deserve it.

Mindwrack Harpy

Flying
At the beginning of combat on your turn, each player mills three cards.

Flying
At the beginning of combat on your turn, each player puts the top three cards of their library into their graveyard.

The List

Commander Masters

Flying
At the beginning of combat on your turn, each player mills three cards. (They each put the top three cards of their library into their graveyard.)