msem-instigator

Whenever Marshback Knights becomes the target of a spell or ability an opponent controls, counter it unless they pay {3}.
{1}{U}: Marshback Knights gains flying until end of turn.

Marshy Vista enters tapped.
{T}: Add {B} or {R}.
{1}, {T}: Add one mana of any color.

Master Assassin can't be blocked by creatures with odd mana values. (Zero is even. This creature's mana value is 2.)
Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on Master Assassin. You may have it phase out.

Whenever an Actor you control becomes untapped, you may discard a card. If you do, draw a card.

As long as you control an Island, you may cast this spell as a copy of any creature on the battlefield. (This includes casting cost.)

Enchant creature
Enchanted creature has defender.

First strike
Mauling Mother gets +1/+1 for each other creature you control.

Whenever you condemn a creature, each opponent sacrifices a creature.
{4}{B}: Judge target creature you control. (Honor it by putting a +1/+1 counter on it or condemn it by sacrificing it to draw a card.)

Flying
Whenever you cast a spell, draw a card, then put a +1/+1 counter on Maven of Perspective.

Your opponents can't gain life.
If a source you control would deal damage to an opponent, it deals double that damage instead.
At the beginning of your end step, if Mayhem Tandem didn't attack this turn, transform it.

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

Memorial Ground enters tapped.
{T}: Add {C}.
{4}, {T}, Sacrifice Memorial Ground: Return target card from your graveyard to your hand.

{T}: Add {C}.
{T}, Exile Memorial of the Fallen: Shuffle your graveyard into your library.

Lifelink
If a source would deal damage to a player, you may prevent X of that damage, where X is the number of +1/+1 counters on Mending Caretaker.

{T}: Add {C}.
Satrapies you control enter untapped.

{T}, Sacrifice Merchant's District: Search your library for an Island or Mountain card and put it onto the battlefield tapped, then shuffle.

({T}: Add {B}.)
As Midnight Gate enters, you may pay 1 life. If you don't, Midnight Gate enters tapped.
Basic lands you control have “{T}: Add {B}.”

{T}: Add {C}.
{3}: Mill two cards, then return Midnight Keep from your graveyard to the battlefield tapped.

MiliCore Barracks enters tapped.
{T}: Add {B} or {R}.
{B}{R}, {T}: Create a Function token with “Deathtouch.” Activate only if an opponent lost life this turn. (It's an enchantment. As you cast an instant or sorcery spell, sacrifice any number of Functions to add their effects to that spell.)

Enchant player
Enchanted player's maximum hand size is reduced by two.

Mind Palace enters tapped.
{T}: Add {U} or {B}.
When Mind Palace enters, you may put a loyalty counter on a planeswalker you control.

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.)
When the fourth spell of a turn is cast, you may unlock Mirror Cipher. If the revealed card is an instant or sorcery card, cast it without paying its mana cost and copy it. You may choose new targets for the copy.

Mirrorgate enters tapped.
When Mirrorgate enters, you choose a nonbasic land card name other than Mirrorgate, then each player chooses a color.
Lands with the chosen name are lands with “{T}: Add one mana of any of the chosen colors,” and lose all other abilities, card types, and land types.
{T}: Add one mana of any of the chosen colors.

Players play with the top card of their libraries revealed.
[+2]: Target player mills a card. Draw a card.
[+1]: Create a 1/1 blue Illusion creature token with “This gets +1/+1 for each other Illusion you control.”
[–4]: Return target creature card from a graveyard to the battlefield under your control. It's an Illusion in addition to its other types.