msem-instigator

When Lifespark Alchemist enters, create a Vial token. (It's an artifact with “{1}, Sacrifice this and another permanent: Draw a card. Activate only during your turn.”)
Whenever Lifespark Alchemist or another creature dies, each opponent loses 1 life.

Advance {2} (Cast this for {2} to advance it as it enters. {2}: Advance this as a sorcery.)
Equipped creature gets +1/+1 for each artifact you control.
Equipped creature must be blocked by an additional creature for each time it and this artifact have advanced this turn to be blocked.
Equip {3}

Enchant creature
Enchanted creature gets +0/+2 and has “{T}: Draw a card.”

When Little Pig enters, if you control a land named Far-Off Lands, put two +1/+1 counters on it.

Menace
When Lord of Decay enters, put a -1/-1 counter on it and each creature you don't control.

Counter target spell, activated ability, or triggered ability. Its controller chooses draw step, main phase, or combat phase. That player skips each instance of the chosen step or phase during their next turn. (Mana abilities can't be targeted.)

Look at the top three cards of your library. Put one of them into your hand. Put Lost in Deep Storage and the rest on the bottom of your library in any order.

Whenever you make first contact, create a 1/1 white Bird creature token with flying. (You make first contact the first time each turn you cast a spell with a name not in your graveyard or control already.)
Whenever three or more creatures you control deal combat damage to a player, create a token that's a copy of another target permanent you control.

When Lunar Pact enters, target creature attacks during its controller's next combat if able.
Players can't untap more than two nonbasic permanents during their untap step.

If Madame Beatrice would leave the battlefield, exile her instead.
Whenever one or more creatures you control deal combat damage to a player, that player reveals their hand. You may cast a card from their hand without paying its mana cost.

Draw a card.
Agonize (You may cast this card from your graveyard by putting a -1/-1 counter on a creature you control in addition to paying its other costs. Then exile this card.)

({T}: Add {W}, {B}, or {G}.)
Majestic Dominion enters tapped.

Remove all counters from target permanent with counters. If it's a creature or enchantment, destroy it.

Whenever Marine Delver attacks, draw a card.

When Mechanical Mastodon enters, you may return another target permanent you control to its owner's hand.

Whenever a player discards a card, you may draw a card and put a loyalty counter on Mei Liva, the Broken.
[0]: Target permanent's controller sacrifices it unless they discard two cards.
[–1]: Each opponent loses X life and you gain X life, where X is the number of cards in your hand.
[–3]: Target player chooses a card in their hand, then discards the rest.

Codex — Unraveling; Unraveling; Unrelenting; Unrelenting; Mindrip; Swipe Secrets. (As this enters, conjure its contents into a shuffled face-up additional library.)
Flash
When Mei's Collection enters, draw a card from its codex.

Enchant creature
When Melt the Chains enters, exile the top two cards of your library. Until the end of your next turn, you may play those cards.
Enchanted creature gets +3/+0.

Activated abilities you control cost up to {1} less to activate. This effect can't reduce the amount of mana an ability costs to activate to less than one mana.

When Militia Surgeon enters, you gain 2 life.
Persist (When this creature dies, if it had no -1/-1 counters on it, return it to the battlefield under its owner's control with a -1/-1 counter on it.)

Sacrifice Misleading Wisp:
Counter target spell unless its controller pays {1}.

Lifelink
At the beginning of your end step, you gain 1 life if you gained life this turn, and each opponent that lost life this turn loses 1 life.

Morningstar Blaze deals X damage divided as you choose among one or two targets.

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.

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