msem-instigator

Senaturian Legion gets +1/+1 for each other creature on the battlefield named Senaturian Legion.
A deck can have any number of cards named Senaturian Legion.

Enchant creature
When Sprawled Arming enters the battlefield, put a +1/+1 counter on enchanted creature.
Enchanted creature has first strike.
{5}: Return Sprawled Arming to its owner's hand.

Whenever you attack, create a white Equipment artifact token with “Equipped creature gets +1/+1” and equip {1}. Attach it to an attacking creature you control.
At the beginning of your end step, untap each embattled creature you control. (A creature is embattled if it has a counter on it or has a permanent you control attached to it.)

Create two 1/2 blue Bird creature tokens with flying.
Coalesce {5}{U}{U} (You may cast this from your graveyard for {5}{U}{U}. If you do, put it onto the battlefield as a Horror creature after it resolves. Exile it if it would leave the battlefield or not resolve.)

Return target nonland permanent to its owner's hand.
Investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
When Dancer of the Great Current enters the battlefield, tap up to one target creature. If you control two or more other creatures that are Merfolk, Wizards, or Rogues, instead return that creature to its owner's hand and put a +1/+1 counter on Dancer of the Great Current.

Draw a card.
Reprise {2} (As you cast an instant or sorcery spell, you may exile one card with reprise from your graveyard and pay its reprise cost. If you do, add its effects to that spell.)

Artifact spells you cast cost {1} less to cast.

Flying
Other creatures you control with flying get +1/+1.

At the beginning of your upkeep, each opponent mills cards equal to the number of card types among permanents you control.

Counter target spell unless its controller pays {1}. If you control a creature with flying, counter that spell unless its controller pays {4} instead.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Whenever Manipulator of Waves attacks, mill three cards. Then create a 0/0 blue Elemental Serpent creature token with “This creature gets +1/+1 for each five cards in your graveyard.”

When Master of the Abyss enters the battlefield, draw two cards, then discard a card.
{2}{U}{U}, {T}, Discard two cards: Create an X/X black Horror creature token, where X is the number of cards in your graveyard.

Prowess
Whenever you cast your first noncreature spell during each of your turns, create a 1/1 blue Reflection Wizard creature token with prowess.

Draw two cards, then discard two cards. Then if you cast Price of Wisdom from your graveyard, create a 3/3 blue Sphinx creature token with flying.
Flashback {4}{U}{U} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Defender (This creature can't attack.)
Whenever you cast a spell during an opponent's turn, scry 1. (To scry 1, look at the top card of your library. You may put that card on the bottom of your library.)

Flash
When Qililia's Scavenger enters the battlefield, look at the top six cards of your library. You may put a Merfolk card with lesser mana value from among them onto the battlefield. Put the rest on the bottom of your library in a random order.
Other Merfolk you control get +1/+1.

Scry 2, then draw a card.
If you control a legendary permanent, investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)

When Rune-Scribe's Apprentice enters the battlefield, target player mills four cards.

This spell can't be countered.
Draw a card.
Reprise {2}{U} (As you cast an instant or sorcery spell, you may exile one card with reprise from your graveyard and pay its reprise cost. If you do, add its effects to that spell.)

Bleed {0} (You may cast this spell for its bleed cost if an opponent has lost life this turn.)
Look at target player's hand.
Draw a card.

You may reveal the second card you draw each turn as you draw it. When you do, create a 2/2 blue Avatar Wizard creature token with prowess if it's a creature card. If it's a land card, create a Treasure token. If it's a noncreature, nonland card, Steward of Second Sphere can't be blocked this turn.
{3}{W/U}{W/U}: Scry 2.

Synchronous Battle Armor and equipped creature get +1/+1 for each artifact you control.
Whenever equipped creature attacks, tap up to three target creatures.
Reconfigure {3}{U}{U} ({3}{U}{U}: Attach to target creature you control; or unattach from a creature. Reconfigure only as a sorcery. While attached, this isn't a creature.)

When Temur Falconer enters the battlefield, create a 1/1 blue Bird creature token with flying.
Whenever a creature you control with flying deals combat damage to a player or planeswalker, put a +1/+1 counter on Temur Falconer.

Whenever you draw a card, you may have target opponent exile the top card of their library face down.
You may look at and play cards exiled with Thought Siphon, and you may spend blue mana as though it were mana of any color to cast them.