msem-instigator

First strike
Creatures you control attacking cause abilities of that creature to trigger as though it entered.

Rice Fields enters tapped.
{T}: Add {W} or {U}.
When Rice Fields enters, you may put a loyalty counter on a planeswalker you control.

Puzzlebox (Begin the game with this and another card you own from outside the game in the command zone, locked. That card is face down until you unlock this.)
{1}: If you control ten or more nonland permanents, unlock Riddle of Entropy.
As long as the revealed card is a permanent card, each nonland permanent you own is a copy of it.

At the beginning of your upkeep, you lose 1 life and choose one —
• Investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)
• You may sacrifice an artifact. If you do, draw a card.

Whenever you cast a creature spell or create a creature token, you may exile target creature you control. Return it to the battlefield under its owner's control at the beginning of the next end step.

Whenever you cast a nonblue spell, create a 1/0 blue Horse Nymph creature token.
Horses get +1/+1 and have flying.

This spell costs {1} less to cast for each artifact and enchantment you control.
Reveal the top seven cards of your library. Put any number of noncreature, nonland cards from among them into your hand and the rest of the revealed cards into your graveyard.

Copy target instant or sorcery spell you control. You may choose new targets for the copy.(Cast a copy of this spell once while this creature is on the battlefield.)

This spell costs {2} less to cast and may be cast as though it had flash if an opponent searched their library this turn.
When Ridge Warden enters, exile another target nonland permanent with mana value 4 or less until Ridge Warden leaves the battlefield.

Target creature gets -2/-2 until end of turn.
{3}{B}, Exile Rift Splicing from your graveyard: Aetherize target creature card with mana value 3 or less in a graveyard. Activate only as a sorcery. (To aetherize, create a token that's a copy of it, then exile that token.)

Flying, menace
When Riftkeeper Entity enters, discard your hand, then draw cards equal to the number of differently named lands you control.
{1}{R}, Discard Riftkeeper Entity: Create a tapped colorless land token named Fading Rift with “{T}, Sacrifice this land: Add one mana of any color.”

{T}: Add {C}.
{3}, {T}: Add {C} for each basic land you control.

Enchant creature
Enchanted creature has “At the beginning of each player's end step, that player chooses another creature they control. That creature fights enchanted creature.”

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.

First strike, lifelink
Tribute 4 (As this creature enters, an opponent of your choice may put four +1/+1 counters on it.)
When Righteous Priestess enters, if tribute wasn't paid, the chosen player can't cast creature spells during their next two turns.

When Rigid Conformist enters, for each {W} spent to cast it, create a 1/1 white Human creature token.
Each creature you control gets +2/+2 if you control another creature with the same name as that creature.

Double strike
Whenever an opponent casts a noncreature spell, Riko, the Paragon deals 2 damage to that player.
Whenever a creature you control becomes blocked, untap it.

Rilken Forcefield Keeper gets +1/+1 for each enchantment you control.
Sacrifice Rilken Forcefield Keeper: Exile target permanent you control. Return that card to the battlefield under your control at the beginning of the next end step.

Whenever another creature you control enters, put a +1/+1 counter on it.
Whenever Rilken Preserver attacks, you may exile target nonland permanent you control, then return it to the battlefield under your control.

{T}: Exile target creature you control.
{2}, {T}: Target creature you control becomes a copy of a card exiled with Ring of Remembrance until end of turn.
{4}, {T}, Sacrifice Ring of Remembrance: Each creature becomes a copy of a card exiled with Ring of Remembrance until end of turn.

As long as equipped creature has power 3 or less, it can't be blocked and it can't be the target of spells and abilities you don't control.
Equip {1}
Whenever Ring of Souls becomes attached to a creature, an opponent of your choice may create a 2/2 black Spirit creature token.

This spell costs {1} less to cast for each instant and/or sorcery card in your graveyard.
Once each turn, when you cast an instant or sorcery spell, you may create a 4/4 white Angel Cleric creature token with flying and vigilance.

This spell can't be countered.
Haste
Whenever a creature dealt damage by Ripper of the Fang Den this turn dies, put X +1/+1 counters on Ripper, where X is his power.

At the beginning of your end step, aetherize target permanent you control. (To aetherize, create a token that's a copy of it, then exile that token.)

Flash
When Rippling Water Arcanist enters, copy target instant or sorcery spell you control, then return it to its owner's hand. You may choose new targets for the copy.