Flying
When Magister Sphinx enters the battlefield, target player's life total becomes 10.

Malefic Scythe enters the battlefield with a soul counter on it.
Equipped creature gets +1/+1 for each soul counter on Malefic Scythe.
Whenever equipped creature dies, put a soul counter on Malefic Scythe.
Equip (: Attach to target creature you control. Equip only as a sorcery.)

When Mana Geode enters the battlefield, scry 1.
: Add one mana of any color.

Manascape Refractor enters the battlefield tapped.
Manascape Refractor has all activated abilities of all lands on the battlefield.
You may spend mana as though it were mana of any color to pay the activation costs of Manascape Refractor's abilities.

When Mangara's Tome enters the battlefield, search your library for five cards, exile them in a face-down pile, and shuffle that pile. Then shuffle your library.
: The next time you would draw a card this turn, instead put the top card of the exiled pile into its owner's hand.

Marble Diamond enters the battlefield tapped.
: Add .

When Mask of Immolation enters the battlefield, create a 1/1 red Elemental creature token, then attach Mask of Immolation to it.
Equipped creature has "Sacrifice this creature: It deals 1 damage to any target."
Equip (: Attach to target creature you control. Equip only as a sorcery.)

When Maul of the Skyclaves enters the battlefield, attach it to target creature you control.
Equipped creature gets +2/+2 and has flying and first strike.
Equip

As Metallic Mimic enters the battlefield, choose a creature type.
Metallic Mimic is the chosen type in addition to its other types.
Each other creature you control of the chosen type enters the battlefield with an additional +1/+1 counter on it.

When Metalspinner's Puzzleknot enters the battlefield, you draw a card and you lose 1 life.
, Sacrifice Metalspinner's Puzzleknot: You draw a card and you lose 1 life.

When Meteor Golem enters the battlefield, destroy target nonland permanent an opponent controls.

When Meteorite enters the battlefield, it deals 2 damage to any target.
: Add one mana of any color.

When Mind Carver enters the battlefield, attach it to target creature you control.
Equipped creature gets +1/+0. It gets +3/+1 instead as long as an opponent has eight or more cards in their graveyard.
Equip

Mindless Automaton enters the battlefield with two +1/+1 counters on it.
, Discard a card: Put a +1/+1 counter on Mindless Automaton.
Remove two +1/+1 counters from Mindless Automaton: Draw a card.

Whenever a nontoken creature enters the battlefield under your control, you may pay . If you do, create a token that's a copy of that creature, except it has haste and "At the beginning of the end step, sacrifice this permanent."

Imprint — When Mirror Golem enters the battlefield, you may exile target card from a graveyard.
Mirror Golem has protection from each of the exiled card's card types. (Artifact, creature, enchantment, instant, land, planeswalker, sorcery, and tribal are card types.)

Mirror Mirror enters the battlefield tapped.
, , Sacrifice Mirror Mirror: Choose target player. At the beginning of the next end step, exchange life totals with that player, exchange control of all permanents you and that player control, and exchange cards in your hands, cards in your libraries, and cards in your graveyards.

As Mirror of the Forebears enters the battlefield, choose a creature type.
: Until end of turn, Mirror of the Forebears becomes a copy of target creature you control of the chosen type, except it's an artifact in addition to its other types.

Reflect (As this enters the battlefield, each opponent may pay . When they do, they create a token copy of this except it lacks this ability.)
, Exile Mirrored Lotus: Add three mana of any one color.

Whenever another nontoken artifact enters the battlefield under your control, you may pay . If you do, create a token that's a copy of that artifact.