msem-instigator

Modular Mercenary gets +1/+0 and has first strike as long as you control a creature with a +1/+1 counter on it.

Gestalt (When this or another creature enters under your control, you may merge it with the one in play, keeping counters and attachments. They become the permanent on top plus all abilities from under it.)
Double strike
If this creature would deal noncombat damage, it deals double that damage instead.

Molting Abomination can't be blocked by creatures with power 2 or less.
Whenever Molting Abomination deals combat damage to a player or planeswalker, create a tapped 3/2 colorless Horror creature token. Put a stun counter on that token. (If a permanent with a stun counter would become untapped, remove one from it instead.)

Monstrous Amalgam gets +X/+X, where X is the number of creature cards in your graveyard.
{B}, Reappropriate: Put a +1/+1 counter on Monstrous Amalgam. Activate only once each turn. (To reappropriate, exile exactly two cards from your hand and/or graveyard.)

First strike

Sacrifice Moonlight Agent: Blocking creatures gain first strike until end of turn.

First strike
Other Harpies you control get +1/+1 and have first strike.
Whenever a Harpy you control dies, put a +1/+1 counter on Moonlight Stalker.

Flying
When Moonlit Charmer enters, tap target artifact or creature. It doesn't untap during its controller's next untap step.

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

When Moonshadow Remnant enters, destroy target tapped creature.
Mirage {3}{W}{W} (You may cast this spell as though it had flash for its mirage cost. If you do, sacrifice it at the beginning of the next cleanup step.)

Flying
{1}{U}: Reveal the top card of your library. Moonwing Watcher's base power becomes equal to that card's mana value.
Prophecy {U} (Once each turn, you may exile a card with prophecy as you reveal it. When you do, you may cast it for its prophecy cost as long as it's exiled.)

Trample
When Moranai Outriders enters, if it's not a token, create a token that's a copy of it, except it has no mana cost.

At the beginning of your upkeep, you may put a reinforcement counter on Moranai Paragon.
Whenever Moranai Paragon attacks, you may put a creature card with mana value equal to the number of reinforcement counters on Moranai Paragon from your hand onto the battlefield.

Moranai Survivalist enters with three +1/+1 counters on it if you didn't cast it from your hand.
Revive {3}{G}{G} ({3}{G}{G}, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

Mortal Tapestry gets +1/+1 for each land card in your graveyard.
Revive {4}{G} ({4}{G}, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

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.

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

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.

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.

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.

When Mouthpiece of Yhzag enters, you may have target opponent create a tapped 1/1 colorless Human creature token.
Whenever Mouthpiece of Yhzag attacks, gain control of target Human creature you don't control with mana value 3 or less until end of turn. Tap it. It's attacking.

Sacrifice a creature or discard a card: Exile the top card of target player's library. You may cast that card for as long as it remains exiled. Any player may activate this ability but only once each turn and only during their turn.
You and permanents you control have hexproof from spells that were exiled with Mr. Jackston or that share a name with a card exiled with Mr. Jackston.

Menace
Whenever Msuji Operative deals combat damage to a player, reveal the top card of your library, put it into your hand, then you lose life equal to its mana value unless you discard a card.

Deathtouch
Respawn (When this creature dies, if it attacked this turn, return it to its owner's hand.)