Millstone Combo with Fraying Sanity for Modern

Hedron Crab triggers whenever a land enters the battlefield under your control, milling three cards from the top of your opponent's library. Pairing this with Thought Scour, which mills two cards and lets you draw a card, enhances the mill strategy. Glimpse the Unthinkable mills ten cards, quickly depleting the opponent's library.

Fraying Sanity causes the opponent to mill additional cards equal to the number of cards they have milled since it entered the battlefield. Archive Trap can be cast for free if an opponent has searched their library, milling 13 cards. Mesmeric Orb mills cards each turn based on how many permanents you tap, significantly boosting the overall milling effect.

Snapcaster Mage allows you to recast an instant or sorcery from your graveyard, effectively reusing Fact or Fiction to dig deeper into your library for more mill spells or answers. Surgical Extraction can remove key cards from your opponent's graveyard or library, disrupting their strategy while benefiting from Snapcaster Mage's ability to bring it back when needed.

Crypt Incursion gains you life equal to the number of creature cards put into a graveyard from anywhere, which can be triggered by your own milling effects. Visions of Beyond allows you to draw a card if there are 20 or more cards in your graveyard, synergizing with the milling strategy and ensuring you keep your hand full.

These lands provide consistent access to blue and black mana, essential for casting the various spells in this mill deck. Polluted Delta can be sacrificed to fetch either Watery Grave or an Island, ensuring you have the right colors to cast your spells while maintaining a low mana curve and supporting your strategy of milling.