msem-instigator

When Thunderwhirl Dancer enters, it gets +X/+X until end of turn, where X is the number of +1/+1 counters among creatures you control. Then it fights up to one target creature.

Creature tokens you control get +1/+1.
Whenever Thyra, Winter's Warden enters or attacks, you may pay {1}{G/W}. If you do, create Bruga, a legendary 2/2 green and white Wolf creature token.

Lionize {1}{U} ({1}{U}, Exile this card from your hand: Put a +1/+1 counter on target creature and it gains the following ability until end of turn. You may cast this card on a later turn from exile.)
Sacrifice this creature: Counter target instant or sorcery spell unless its controller pays {X}, where X is this creature's power.

Vanishing 4 (This creature enters with four time counters on it. At the beginning of your upkeep, remove a time counter from it. When the last is removed, sacrifice it.)
Remove a time counter from Tideshift Elemental: You may tap or untap target creature.

Once each turn, when a creature you control becomes tapped, you may untap that creature and tap up to one target creature.

Enchant creature
When Til Death enters, exile target creature an opponent controls other than enchanted creature until Til Death leaves the battlefield.
Enchanted creature has defender.

Each opponent sacrifices an artifact or enchantment.
Showcase {2}{G}{G} (Then if you cast this for its showcase cost, create a 1/1 red Bard creature token that can't block and exile this. Whenever it attacks, you may cast a free copy of this.)

Target creature you control gets +4/+4 until end of turn.
Compose (Then if you cast this from your hand, exile it and create a Saga token with “r3—You may cast a copy of this card from exile without paying its mana cost.”)

When you next cast an instant or sorcery spell this turn, copy that spell. You may choose new targets for the copy.
Compose (Then if you cast this from your hand, exile it and create a Saga token with “r3—You may cast a copy of this card from exile without paying its mana cost.”)

If you have less life than an opponent, you gain 6 life. If you control fewer creatures than an opponent, create three 1/1 white Soldier creature tokens.

Whenever a creature you control enters, Timid Thunderhorn gets +X/+X until end of turn, where X is that creature's power.

You may pay {1}{G} and tap an untapped Soldier you control rather than pay this spell's mana cost.
Put two +1/+1 counters on target creature. It gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

Return target creature card from your graveyard to the battlefield, then destroy target creature an opponent controls that doesn't share a creature type with it.

Flying
Tribute 2 (As this creature enters, an opponent of your choice may put two +1/+1 counters on it.)
When Tireless Vulture enters, if tribute wasn't paid, each opponent loses 3 life. You gain life equal to the life lost this way.

Prowess
Whenever you cast an instant or sorcery spell, Tirenan Pyromancer deals 1 damage to each opponent.

When Titan of Lake Vasil enters, untap up to four target permanents.
{3}{U}: Draw a card.

Colossal 2—{6}{U} (You may cast this for its colossal cost to have it enter with two +1/+1 counters; or pay {6}{U} as a sorcery: Put two +1/+1 counters on it if it has none.)
When Titanic Fathomclaw's colossal ability puts one or more counters on it, return target instant or sorcery card from your graveyard to your hand.

First strike
Noncreature spells with mana value 4 or greater cost {1} more to cast.

When Titou Louoe, Bayou Shaman enters, create a blue Aura Curse enchantment token named Curse of Impotence attached to target player. The token has enchant player and “Creatures enchanted player controls get -1/-0.”

Until end of turn, target creature gets +1/+0 and gains “Whenever this creature is dealt damage, exile that many cards from the top of your library. Until end of turn, you may play those cards.”

Arm target creature you control. It deals damage equal to its power to any target. (Create a colorless Equipment artifact token with “Equipped creature gets +1/+0” and equip {2}, then attach it to that creature.)

Creatures with power 4 or greater can't attack or block unless their controller pays {2} for each of those creatures.

Whenever a creature you control attacks, you may sacrifice a Treasure. If you do, that creature can't be blocked this turn.

You may put cards with mana value 2 or less that permanents you control explore into your hand as though they were lands. (You won't get the counter if you do.)
Whenever Torch of Those Underfoot attacks, it explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

Attacking creatures you control get +1/+0.
Mirage {2}{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.)