Cast Maddening Cacophony with kicker by paying {4}{U}{U}, causing each opponent to mill half their library, rounded up.
Activate Keening Stone by paying {5} and tapping it, causing an opponent to mill cards equal to the number of cards in their graveyard, thus milling the rest of their library.

Infinite mill for target opponent

Near-infinite mill

Maddening Cacophony

Keening Stone

Cast Traumatize by paying {3}{U}{U}, causing an opponent to mill half their library, rounded down.
Activate Keening Stone by paying {5} 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.

Traumatize

Cast Cut Your Losses by paying {4}{U}{U}, causing an opponent to mill half their library, rounded down.
Activate Keening Stone by paying {5} 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.

Cut Your Losses

Declare Lord Xander as an attacker, triggering its second ability, causing a player to mill half their library, rounded up.
Activate Keening Stone by paying {5} 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.

Infinite self-mill

Fleet Swallower

Declare Terisian Mindbreaker as an attacker, triggering its ability, causing the defending player to mill half their library, rounded up.
Activate Keening Stone by paying {5} 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.

Terisian Mindbreaker

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} 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.

Combos with "Keening Stone"