Millstone

• WWizards of the Coast2004-10-04
  Official Ruling
  Since you are putting more than one card in the graveyard at one time, the affected player can choose the order the 2 cards go in.
• WWizards of the Coast2004-10-04
  Official Ruling
  Can be used on a player with less than 2 cards in their library. It will remove 0 or 1 cards if that is all that is available.
• WWizards of the Coast2004-10-04
  Official Ruling
  It is not a draw effect so it will not cause a player with less than 2 cards in their library to lose.

Rulings via Scryfall