If an opponent would mill one or more cards, they mill twice that many cards instead. (To mill a card, a player puts the top card of their library into their graveyard.)
1 /
4
{1}{U}
Drowned Secrets
Enchantment
Whenever you cast a blue spell, target player mills two cards.
{5}{U}{U}
Fleet Swallower
Creature - Fish
Whenever Fleet Swallower attacks, target player mills half their library, rounded up.
6 /
6
{2}{U}
Fraying Sanity
Enchantment - Aura Curse
Enchant player
At the beginning of each end step, enchanted player mills X cards, where X is the number of cards put into their graveyard from anywhere this turn.
{1}{U}{U}
Memory Erosion
Enchantment
Whenever an opponent casts a spell, that player mills two cards.
{3}{U}{U}
Patient Rebuilding
Enchantment
At the beginning of your upkeep, target opponent mills three cards, then you draw a card for each land card put into their graveyard this way.
{2}{U}
Psychic Corrosion
Enchantment
Whenever you draw a card, each opponent mills two cards.
{2}{U}
Sphinx's Tutelage
Enchantment
Whenever you draw a card, target opponent mills two cards. If two nonland cards that share a color were milled this way, repeat this process.
{5}{U}: Draw a card, then discard a card.
{2}{U}
Teferi's Tutelage
Enchantment
When Teferi's Tutelage enters the battlefield, draw a card, then discard a card.
Whenever you draw a card, target opponent mills two cards. (They put the top two cards of their library into their graveyard.)
{3}{U}{U}
Traumatize
Sorcery
Target player mills half their library, rounded down.
{1}{U}
_____
Creature - Shapeshifter
{1}: This card's name becomes the card name of your choice. Activate anywhere, anytime.