msem-instigator

Flying
When Horned Spy enters, look at target player's hand.

{2}{U}{B}, Sacrifice a creature: Return target creature card from your graveyard to the battlefield with a finality counter on it. It gains myriad. (Whenever it attacks, for each opponent other than defending player, you may put a token that's a copy of this creature onto the battlefield tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.)

Whenever you cast a storied spell, you may pay {1}. If you do, draw a card. (Enchantments and legendary spells are storied.)
Sacrifice Right to Rule: Storied permanents you control gain indestructible until end of turn.

Gestalt (When this or another creature enters under your control, you may merge it with the one in play, keeping counters and attachments. They become the permanent on top plus all abilities from under it.)
Whenever this creature attacks, if it's merged with another card or token, it deals 2 damage to target player or planeswalker.

Flying
{1}{R}{R}{R}: Return Umbral Hunter from your graveyard to your hand. If it's the declare attackers step, up to one target attacking creature gets +3/+1 until end of turn.

Target creature gets +3/+3 until end of turn and must be blocked this turn if able.
Showcase {3}{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.)

Whenever equipped creature attacks, it deals damage to defending player equal to the number of instant and sorcery cards in your graveyard.
Equip {2}

{2}, {T}: Draw a card. Crude Tablet doesn't untap during its controller's next untap step.

When Shield of the Unwavering enters, attach it to target creature you control.
Equipped creature gets +2/+2 and has vigilance.
Equip {3}

Flying
{2}{W}, Exile a non-Spirit creature card from your graveyard: Create a 1/1 white Spirit creature token with flying.

Rosesworn Knight gets +1/+1 as long as it's embattled. (A creature is embattled if it has a counter on it or a permanent you control attached to it.)

Search your library for an enchantment card, reveal it, put it into your hand, then shuffle.

Attacking creatures have winnow. (Damage dealt to a creature with winnow also causes its controller to lose that much life.)

Enchant creature
Enchanted creature gets +1/+1 and has flying and prowess.

Tribute 3 (As this creature enters, an opponent of your choice may put three +1/+1 counters on it.)
When Atuum Serpent enters, if tribute was paid, it gains defender. (This effect lasts indefinitely.)

Ammbradu Parasite gets +1/+1 as long as you control a Forest.
{4}{G}: Target creature must be blocked this turn if able.

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.)
United Patrol can block an additional creature each combat.

When Indomitable Duo enters, choose up to two creature card names.
Whenever Indomitable Duo attacks, put a +1/+1 counter on each creature you control with a name chosen by Indomitable Duo.

Flash
When Reinforced Platemail enters, you may attach it to target creature you control.
Equipped creature you control gets +1/+2.
Equip {3}

When Soul of the Sands enters, choose one —
• You gain 4 life.
• If you gained life this turn, draw a card.

Sculpted Observatory enters tapped.
{T}: Add {W} or {U}.
{1}, {T}: Add one mana of any color.

When Festival Celebrants enters, put a +1/+1 counter on each creature you control.

Whenever a player casts an instant or sorcery spell that targets exactly one creature they don't control, you may copy that spell. If you do, you may choose new targets for the copy.

When Relearn History enters, draw two cards.
You have no maximum hand size.