msem-instigator

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.)

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.

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.

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}.

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.

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.

Players can't get counters.
Counters can't be put on permanents.

Counter target enchantment, instant, or sorcery spell. (Then exile this card. You may cast the creature later from exile.)

When Night Howlers enters, target opponent reveals three cards from their hand and you choose one of them. Exile that card until Night Howlers leaves the battlefield.

Enchant player
At the beginning of enchanted player's end step, they lose 2 life unless they discard a card.

When Night Shepherd enters, return a creature you control to its owner's hand.

Choose X. Target player reveals their hand and discards all nonland cards with mana value X.

Wither (This deals damage to creatures in the form of -1/-1 counters.)
{1}{B}: Return Noonfiend from your graveyard to your hand.

Exile up to two target creatures controlled by different players. Return those creatures to the battlefield under their owners' control at the beginning of the next end step.

When Ominous Presence enters, search your library for a Curse card, reveal it, then shuffle. If it has mana value 4 or less, you may put it onto the battlefield. Otherwise, put it into your hand.
Ominous Presence gets +1/+1 for each Curse attached to your opponents.

Opera House enters tapped.
When Opera House enters, add one mana of any color.
{T}, Sacrifice Opera House: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Whenever you make first contact, Oscuran Wingmate gains flying until end of turn. (You make first contact the first time each turn you cast a spell with a name not in your graveyard or control already.)

All creatures able to block Osheen Courtier do so.
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.)