msem-instigator

Whenever Moonlit Lurker blocks, put a +1/+1 counter on it.

Moonlit Shore enters tapped unless you control two or more blue and/or black permanents.
{T}: Add {U} or {B}.

{T}: Add {C}.
{T}, Exile two cards from your graveyard: Add {U} or {B}.

Moonshard Crater enters tapped.
{T}: Add {R}.
{2}{R}, {T}: Each player sacrifices a nonbasic land.

{T}: Add {W}.
{W}, {T}: Target creature gains vigilance until end of turn.

{T}: Add {C}.
{1}, {T}, Sacrifice Morphic Verge: Choose two different basic land types. Create a colorless land token that's the chosen types.

Prowess, trample
When this creature transforms into Mortal Frame, add {R}{R}{R}.

Mortal Realms enters tapped unless you control two or more other lands.
{T}: Add {R}.
{T}, Pay 1 life: Add {B} or {G}.

Puzzlebox (Begin the game with this and another card you own from outside the game in the command zone, locked. That card is face down until you unlock this.)
{3}: Unlock Mortal's Conundrum. If the revealed card is black, put it into your hand. Otherwise, put it on top of your library.
At the beginning of your upkeep, if Mortal's Conundrum is unlocked, you lose 2 life.

Mosecf, the Sunseeker's Path enters tapped.
Whenever a creature you control attacks, put a creation counter on Mosecf.
{T}, Remove ten creation counters from Mosecf and sacrifice it: Creatures you control get +2/+2 and gain vigilance until end of turn. Activate only as a sorcery.

({T}: Add {B} or {G}.)
Mossy Shrine enters tapped unless you control two or more basic lands.

Experience (Whenever this creature attacks, if it has fewer than three +1/+1 counters on it, put a +1/+1 counter on it.)

(Transform this if it's not attached to a creature. Pay {1}{W} as a sorcery to unattach it.)
Enchant creature you control
Mount of Heroes's rider gets +2/+1.
Sacrifice Mount of Heroes: Counter target spell that targets you or Mount of Heroes's rider.

(Transform this if it's not attached to a creature. Pay {3}{R}{R} as a sorcery to unattach it.)
Enchant creature you control
Mount of the Order's rider gets +5/+4.
{2}{R}: Target creature can't block Mount of the Order's rider this turn.

Menace
At the beginning of your end step, Mountain Tyrant deals 1 damage to each creature you don't control.

Sacrifice another colorless creature: Put a +1/+1 counter on target creature.

({T}: Add {U} or {B}.)
Muggy Mire enters tapped unless you control two or more basic lands.

Multipurpose Mannequin gets +1/+1 for each permanent attached to it.

Murky Path enters tapped unless you control one or fewer other lands.
{T}: Add {U} or {B}.
Cycling {2} ({2}, Discard this card: Draw a card.)

{T}, Pay 1 life: Add {B}.
{T}, Pay 1 life, Sacrifice Murky Peat: Search your library for a Swamp card, put it onto the battlefield tapped, then shuffle.

({T}: Add {U} or {B}.)
Whenever Murmuring Falls becomes tapped, you lose 1 life unless you control two or more basic lands.

({T}: Add {G} or {U}.)
As Mushroom Lagoon enters, you may pay {1}. If you don't, Mushroom Lagoon enters tapped.

{T}: Add {C}.
Whenever Mustering Grounds becomes tapped, if it's enchanted, you may pay {2}. If you do, create a 1/1 white Soldier creature token with vigilance.

You may have Mutalithic Golem enter as a copy of any land on the battlefield, except it's an artifact in addition to its other types.