msem-instigator

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.

Flying
{U}: Ripclaw Drake gets +1/-1 until end of turn.

Trample
Risti Assault Squadron can't attack unless all other creatures you control also attack. (It can attack if it's the only creature you control.)

Flying

{U}, {T}: You may tap or untap another target creature.
Whenever one or more other creatures you control deal combat damage to a player, you may untap Riverchase Diver.

Harmony — Untap target permanent.
Whenever you cast an instant or sorcery spell, harmonize. (Trigger up to one harmony of one permanent you control.)

{7}{U}: Tap up to three target nonland permanents. This ability costs {2} less to activate if you control a legendary creature.

Flying
Whenever Rivha and at least two other creatures attack, exile all other attacking creatures you control. For each creature exiled this way, create a 3/3 white Angel creature token with flying that's tapped and attacking.

At the beginning of combat on each of your turns, you may sacrifice a land or exile a land card from your graveyard. If you do, create two 1/1 red Goblin creature tokens with haste. Exile those tokens at the beginning of the next end step.

Deploy 2 (As this enters, you may tap any number of other creatures you control with total power 2 or more.)
When Rogue Groundskeeper enters deployed, exile up to two target cards from a single graveyard.

When Rootbound Awakener enters, create a green Aura enchantment token named Vivification attached to target creature or land. The token has enchant creature or land and “Enchanted permanent is a creature with base power and toughness 5/5 in addition to its other types.”

At the beginning of each upkeep, choose another permanent with the greatest mana value among permanents. Its controller sacrifices it.

Rosegill Rover can't be blocked.
Whenever Rosegill Rover attacks, you may discard a card. If you do, draw a card.

({T}: Add {R}.)
Rousing Rubble enters tapped.
Double strike
{2}{R}{R}: Rousing Rubble becomes a creature in addition to its other types until end of turn.

Whenever Roving Dreadnought attacks, you may salvage a card that was put into your graveyard this turn. (Exile one from your graveyard until this creature leaves the battlefield. Whenever this creature deals combat damage to a player, put a card it salvaged into your hand.)

Each Rogue you control can't be blocked by creatures with greater power than it.
Whenever a Rogue deals combat damage to a player, gain control of target nonland permanent that player controls unless that player pays {1}.

Trample
Whenever a noncreature permanent is put into a graveyard from the battlefield, put a +1/+1 counter on Ruinous Naga.

Defender, flying
Ruins Guardian can attack as though it didn't have defender as long as a creature died under your control this turn.
Whenever Ruins Guardian attacks, you may exile two creature cards from your graveyard. If you do, draw a card.

Ward {2}
{G}{G}{G}, {T}, Exile a creature card from your graveyard: Search your library for up to two creature cards that share a name with the exiled card, reveal them, put them into your hand, then shuffle.

When Runebreaker Mage enters, return another target nonland permanent you control and target nonland permanent you don't control to their owner's hands.

Whenever you make first contact, Runic Enforcer gains lifelink until end of turn. (You make first contact the first time each turn you cast a spell with a name not in your graveyard or control already.)

Alpha (You may have this creature enter with a +1/+1 counter on it. If you do, it becomes alpha and can only attack alone.)
Ruthless Predator can't be blocked by more than one creature.

If a player taps a nonbasic land for mana, that land produces {C} instead of any other type.
Whenever Rylei Wakebreaker attacks, it and another target creature you control each get +1/+0 until end of turn for each attacking creature you control.