msem-instigator

Once each turn, when another creature dies, you may crystallize. (Put a +1/+1 counter on a creature you control or create a Treasure.)
Whenever an artifact you control dies, Nihilistic Shock Trooper deals 1 damage to each opponent.

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
When Nizev Brawler enters, target creature can't block this turn.

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
At the beginning of your end step, if you've cast three or more spells this turn and Nizev Enforcer isn't a token, create a token that's a copy of it.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Whenever you cast an enchantment, instant, or sorcery spell, Noble Barracuda gets +1/+0 until end of turn. Scry 1.

Flying
Menagerie — When Northrime Messenger enters, if there are five or more creature types among creatures you control, you may search your library for a Quest card, reveal it, put it into your hand, then shuffle.

When Norwick Barkeep enters, target creature you control gets +1/+1 and gains vigilance until end of turn.

Flash
When Notorious Smuggler enters, target opponent reveals their hand. Choose a nonland card revealed this way. Copy it. You may cast the copy this turn. (You still pay its costs, including color. Copies of permanents enter as tokens.)

Whenever you cast a noncreature spell, Novice Pyromancer deals 1 damage to target player or planeswalker.

When Noxan Zealot enters, create a tapped 5/5 black Demon enchantment creature token.
{T}, Pay 3 life: Target enchantment creature gains menace and deathtouch until end of turn. Activate only as a sorcery.

Abilities of creatures you activate that aren't mana abilities cost {2} less to activate. This effect can't reduce the mana in that cost to less than one mana.
{T}: Target creature gains haste until end of turn.

Flying, deathtouch

Whenever you cast a cryptic spell, Oasis Cattle gets +1/+1 until end of turn, then untap it. (Artifacts, enchantments, and modal spells are cryptic.)

Flash
Whenever you cast an artifact or enchantment spell, put a +1/+1 counter on Observer of Secret Paths. It can't be blocked this turn.

Tribute 2 (As this creature enters, an opponent of your choice may put two +1/+1 counters on it.)
When Observing Guardian enters, if tribute wasn't paid, create a 2/2 white Cat creature token.

Other creatures you control get +2/+0.
Creatures your opponents control get -1/-1.
Fleeting {2}{B} (You may cast this spell for its fleeting cost. If you do, sacrifice it at the beginning of the next end step.)

Flash
When Odaya Magistrate enters, you may counter target noncreature spell.

Deception {2}{U} ({2}{U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever a creature you control deals combat damage to an opponent, you may draw a card.

Other Merfolk you control get +1/+1.
When Olantin Pearlreader enters, scry 2. Then draw a card if you control two or more other Merfolk.

Whenever a creature with power 4 or greater you control enters, it gains haste until end of turn.
{T}: Add {R} or {G}.
{4}{R}{G}: Create a 4/4 red Dragon creature token with flying.

Flying
At the beginning of your end step, untap target permanent you control.

Oldgrove Nurturer enters with a +1/+1 counter on it.
{G}, Remove a +1/+1 counter from a creature you control: Create a 1/1 colorless Construct artifact creature token.

Flying
Whenever Olson's Armada attacks, if only one creature crewed it, create a 1/1 white Bird creature token with flying. Otherwise, put a +1/+1 counter on Olson's Armada.
Crew 2 (Tap any number of creatures you control with total power 2 or greater: This Vehicle becomes an artifact creature until end of turn.)

Whenever you attack with one or more Pirates, create that many 1/1 white Bird creature tokens with flying that are tapped and attacking. At end of combat, choose one of those tokens, then exile the rest.

Cadence (Whenever you cast a spell, if its mana value is greater than this creature's power, put a +1/+1 counter on this creature.)
Creatures with power less than Olten Survivalist's power can't block creatures you control.

Flash
When Olten Vipermoa enters, you may return another creature you control to its owner's hand. If you do, put two +1/+1 counters on Olten Vipermoa.