Zellix, Sanity Flayer

Hive Mind — Whenever a player mills one or more creature cards, you create a 1/1 black Horror creature token.
{1}, {T}: Target player mills three cards.
Choose a Background (You can have a Background as a second commander.)

Perpetual Timepiece

{T}: Mill two cards.
{2}, Exile Perpetual Timepiece: Shuffle any number of target cards from your graveyard into your library.

Brain Freeze

Target player mills three cards.
Storm (When you cast this spell, copy it for each spell cast before it this turn. You may choose new targets for the copies.)

Codex Shredder

{T}: Target player mills a card. (They put the top card of their library into their graveyard.)
{5}, {T}, Sacrifice Codex Shredder: Return target card from your graveyard to your hand.

Emry, Lurker of the Loch

This spell costs {1} less to cast for each artifact you control.
When Emry, Lurker of the Loch enters, mill four cards.
{T}: Choose target artifact card in your graveyard. You may cast that card this turn. (You still pay its costs. Timing rules still apply.)

Slogurk, the Overslime

Trample
Whenever a land card is put into your graveyard from anywhere, put a +1/+1 counter on Slogurk, the Overslime.
Remove three +1/+1 counters from Slogurk: Return it to its owner's hand.
When Slogurk leaves the battlefield, return up to three target land cards from your graveyard to your hand.

Hedron Crab

Landfall — Whenever a land you control enters, target player mills three cards.

Mesmeric Orb

Whenever a permanent becomes untapped, that permanent's controller mills a card.

Millikin

{T}, Mill a card: Add {C}. (To mill a card, put the top card of your library into your graveyard.)

Drown in Dreams

Choose one. If you control a commander as you cast this spell, you may choose both instead.
• Target player draws X cards.
• Target player mills twice X cards.