msem-instigator

Players can't gain life.
At the beginning of each player's upkeep, if that player has 10 or less life, they lose 1 life.

Whenever Mortified Medium or another creature enters, you gain 1 life.
As long as you have at least 10 life more than your starting life total, Mortified Medium gets +4/+4 and has flying.

Flying
{1}{W/U}{W/U}: Creatures you control gain flying until end of turn if another creature you control has flying. The same is true for first strike, double strike, deathtouch, haste, indestructible, lifelink, menace, reach, trample, and vigilance.

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.

When Moss Dweller enters, put a +1/+1 counter on another target creature.

Deathtouch

Mossvine Lurker has trample as long as it has a +1/+1 counter on it.

Vigilance, reach, trample

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

When Moth-Heart Husk dies, create a 1/1 black Spirit creature token with flying.
Revive {3}{B} ({3}{B}, Exile this card from your graveyard: Create a token that's a copy of this card, except it has no mana cost. Activate this ability only any time you could cast a sorcery.)

Deathtouch
Other creatures you control get -1/-1.
Whenever Mother Night attacks, put target creature card from a graveyard onto the battlefield under your control.

{B}{G}{U}: Mother Roo gets +2/+2 and gains flying until end of turn.

Flying
Whenever another creature you control enters, you gain 1 life.

Whenever you sacrifice a permanent, you may put a -1/-1 counter on target creature.

Flying
When Mother of Owls dies, you may return target creature an opponent controls to its owner's hand.

Whenever a creature dies, put a +1/+1 counter on Mother's Disciples.

Motley Crew has no name while it's being cast. (It will always make first contact.)
{2}{R}: Creatures you control named Motley Crew get +1/+0 until end of turn.

{4}{G}: Monstrosity 2. (If this creature isn't monstrous, put two +1/+1 counters on it and it becomes monstrous.)
When Mountain Bighorn becomes monstrous, you may have it fight target creature you don't control.

Whenever a land you control enters, put a +1/+1 counter on Mountain Runt, then it deals 1 damage to up to one target creature you don't control.
When Mountain Runt has three or more +1/+1 counters on it, remove them all and transform it.

Kicker {2}{R} (You may pay an additional {2}{R} as you cast this spell.)
Haste (This creature can attack and {T} as soon as it comes under your control.)
If Mountain Victor was kicked, it enters with two +1/+1 counters on it.

When Mountain-Western Inventor enters, you may return target artifact card from your graveyard to your hand.
{1}, Sacrifice an artifact: Target creature can't block or be blocked this turn.

Creatures you control get +2/+2 and gain first strike until end of turn.

First strike, haste
When Mounted Marksman enters, it deals 2 damage to target creature you don't control.

At the beginning of combat on your turn, put a +1/+1 counter on target creature you control. If that creature has flying, transform Mounting Up.