Lord Xander, the Collector | Keening Stone

Cards

Lord Xander and
Keening Stone on the battlefield (
Lord Xander without summoning sickness).

Declare
Lord Xander as an attacker, triggering its second ability, causing the defending player to mill half their library, rounded down.
Activate
Keening Stone by paying ({5} magic symbol) and tapping it, causing the player to mill cards equal to the number of cards in their graveyard, thus milling the rest of their library.

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.