Cut Your Losses by paying ({4} magic symbol) ({U} magic symbol) ({U} magic symbol) , causing an opponent to mill half their library, rounded down.
Activate
Keening Stone by paying ({5} magic symbol) and tapping it, causing the opponent to mill cards equal to the number of cards in their graveyard, thus milling the rest of their library.
Notes
If the opponent has an odd number of cards in their library and an empty graveyard, the opponent will have one card remaining in their library. Otherwise they will mill their entire library. You can also pay the casualty cost if desired.