msem-instigator

When Rat King enters, create two 1/1 black Rat creature tokens with “This creature can't block.”
Whenever Rat King or another creature you control dies, each opponent loses 1 life and you gain 1 life.

Whenever a creature you control dies, each opponent discards a card.
Revive {6}{B} ({6}{B}, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

Whenever Ravencaller or another creature enters from your graveyard, create a 1/1 blue Bird creature token with flying.

When Ravenous Chupacabra enters, destroy target creature an opponent controls.

Double strike

Flying
Whenever an opponent discards one or more cards, fateseal 1. (To fateseal 1, look at the top card of an opponent's library, then you may put it on the bottom of that player's library.)

When Razorshell Marauder enters, you may sacrifice a land or exile a land card from your graveyard. When you do, target creature gets -1/-1 until end of turn and can't block this turn.

Trample
When Reckless Exemplar attacks, if its power is greater than its base power, target creature an opponent controls can't block this turn.

Haste
Whenever Reckless Pyremage becomes tapped, it deals 1 damage to each creature and each player.

{2}{B}: Reclaimed Kalak gains indestructible until end of turn.
Revive {3}{W} ({3}{W}, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

Reclusive Copse enters tapped.
{T}: Add {B} or {G}.
{1}{B}{G}: Until end of turn, Reclusive Copse becomes an Insect creature with deathtouch in addition to its other types.
Scavenge {B}{G}

When Reconnaissance Expert enters, mill four cards.
When Reconnaissance Expert dies, return target noncreature permanent card from your graveyard to your hand.

Flash
Prowess
When Reconstructed Adept enters, target instant or sorcery card in your graveyard gains agonize until end of turn. (You may cast it by putting a -1/-1 counter on a creature you control in addition to paying its other costs. Then exile that card.)

Deception {U} ({U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever a Pirate or Rogue you control deals combat damage to a player, rediscover a Pirate or Rogue card. (Look at the top card of your library. You may reveal it and put it into your hand if it's a Pirate or Rogue. Otherwise, put it back or into your graveyard.)

When Recruits of the Demeter enters, draw a card if you control another Pirate.

Whenever Recurring Abomination deals combat damage to an opponent, you may put target creature card with mana value 3 or less from a graveyard onto the battlefield under your control.
{4}{B}{B}: Return Recurring Abomination from your graveyard to the battlefield tapped.

When Recursion Runner enters, create a Function token with “If this spell isn't a copy, copy it. You may choose new targets for the copy.” (It's an enchantment. As you cast an instant or sorcery spell, sacrifice any number of Functions to add their effects to that spell.)

When Red Flower Assassin enters, destroy target creature.
Revive {5}{B} ({5}{B}, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

Other creatures you control get +0/+1.
{T}: Add {G}.

Redleather Armorer gets +1/+1 as long as it's your turn.

Trample
{1}{B}: Rehis-Dorahn Plowbeast gets +1/+1 until end of turn.

Deathtouch

Rehison Soothemage has lifelink as long as you control another Elf.

Reach
{2}{G}: Rei's Ranger gets +2/+0 until end of turn.

When Reincarnation Savant dies, manifest the bottom card of your library. (To manifest a card, put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.)