msem-instigator

At the beginning of your upkeep, discard a card and create a Vial token. (It's an artifact with “{1}, Sacrifice this and another permanent: Draw a card. Activate only during your turn.”)

When Weeping Yaoguai enters, create a 1/1 colorless Spirit creature token.
Scavenge {3}{B} ({3}{B}, Exile this card from your graveyard: Put a number of +1/+1 counters equal to this card's power on target creature. Scavenge only as a sorcery.)

Put two +1/+1 counters on up to one target creature.
Coalesce {6}{G}{G} (You may cast this from your graveyard for {6}{G}{G}. If you do, it enters as a Horror creature after it resolves. Exile it if it would leave the battlefield or not resolve.)

Flying
Welkin Tern can block only creatures with flying.

Whenever West Ward Purifier becomes tapped, you gain 1 life.

Flash
Heretic (Whenever you cast a spell that doesn't share a color with this creature, it gets +1/+1 until end of turn.)

Whenever two or more creatures you control deal combat damage to a player, you lose 1 life and draw a card.

When Whisper Witch enters, choose up to one target creature. That creature's controller mills four cards, then puts that creature on top of their library.

Menace (This creature can't be blocked except by two or more creatures.)

Prowess, haste
Mirage {R} (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.)

Vigilance

You may exert Wiedfree Berserker as it attacks. When you do, target creature can't block this turn. (An exerted creature won't untap during your next untap step.)
If exerting a creature you control causes a triggered ability of a permanent you control to trigger, that ability triggers another time.

Whenever Wild Dogs of the Night or another Horror you control enters, exile up to one target card in a graveyard. If that card was a creature card, you gain 2 life and put two +1/+1 counters on that Horror.

Frenzy 1 (Whenever this creature attacks and isn't blocked, it gets +1/+0 until end of turn.)
Wild Haploc has first strike as long as it's attacking.

Colossal 4—{5}{G} (You may cast this for its colossal cost to have it enter with four +1/+1 counters; or pay {5}{G} as a sorcery: Put four +1/+1 counters on it if it has none.)
Trample

Improve {G} ({G}: Put a +1/+1 counter on this creature. This costs {1} more to activate for each +1/+1 counter on it. Improve only as a sorcery.)
If a +1/+1 counter would be put on a creature you control, you may put a deathtouch, reach, trample, or vigilance counter on it instead.

Assault {2}{G} (Whenever this attacks, if it has no +1/+1 counters on it, you may pay its assault cost to put a +1/+1 counter on it.)
When Wildborn Marauder assaults, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Harmony — You may pay {1}{G}. If you do, create a 3/3 green Beast creature token.
When Wildheart Chorister enters, harmonize. (Trigger up to one harmony of a permanent you control.)

Trample
As long as you control an Elemental creature, Wildheart Gorran gets +1/+1.

{1}{G}, Remove a -1/-1 counter from a creature you control: Wiltthorn Elemental gets +2/+2 until end of turn.

Flash (You may cast this spell any time you could cast an instant.)
When Wind-Blessed Guardian enters, you may return target permanent you control to its owner's hand.

Flying
Revive {1}{B}{B} ({1}{B}{B}, 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.)

Flash
When Wiretapper enters, choose one —
• Exile target spell. For as long as that card remains exiled, its owner may play it. A spell cast this way costs {2} more to cast.
• Create a copy of the card with target spell's name. You may cast the copy. (You still pay its costs.)

Flying