Plains

Benalish Cavalry

Flanking (Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.)

Vodalian Wave-Knight

Whenever you draw a card, put a +1/+1 counter on each other Merfolk and/or Knight you control.

Time of Ice

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I, II — Tap target creature an opponent controls. It doesn't untap during its controller's untap step for as long as you control Time of Ice.
III — Return all tapped creatures to their owners' hands.

Emperor Mihail II

You may look at the top card of your library any time.
You may cast Merfolk spells from the top of your library.
Whenever you cast a Merfolk spell, you may pay {1}. If you do, create a 1/1 blue Merfolk creature token.

Island

Master of the Pearl Trident

Other Merfolk creatures you control get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)

Counterspell

Snow-Covered Plains

General Jarkeld

{T}: Choose two target blocked attacking creatures. If each of those creatures could be blocked by all creatures that the other is blocked by, each creature that's blocking exactly one of those attacking creatures stops blocking it and is blocking the other attacking creature. Activate only during the declare blockers step.

Chromatic Sphere

{1}, {T}, Sacrifice Chromatic Sphere: Add one mana of any color. Draw a card.

Flooded Strand

{T}, Pay 1 life, Sacrifice Flooded Strand: Search your library for a Plains or Island card, put it onto the battlefield, then shuffle.

Love Song of Night and Day

Read ahead (Choose a chapter and start with that many lore counters. Add one after your draw step. Skipped chapters don't trigger. Sacrifice after III.)
I — You and target opponent each draw two cards.
II — Create a 1/1 white Bird creature token with flying.
III — Put a +1/+1 counter on each of up to two target creatures.

Argivian Cavalier

Enlist (As this creature attacks, you may tap a nonattacking creature you control without summoning sickness. When you do, add its power to this creature's until end of turn.)
When Argivian Cavalier enters the battlefield, create a 1/1 white Soldier creature token.

Deliver

Return target permanent to its owner's hand.

Stand

Prevent the next 2 damage that would be dealt to target creature this turn.

Stand // Deliver

Khod, Etlan Shiis Envoy

All lands are Islands in addition to their other types.
Other Homarids, Camarids, Cephalids, Nautilids, and Merfolk you control get +1/+1.

Daring Archaeologist

When Daring Archaeologist enters the battlefield, you may return target artifact card from your graveyard to your hand.
Whenever you cast a historic spell, put a +1/+1 counter on Daring Archaeologist. (Artifacts, legendaries, and Sagas are historic.)

Orim, Samite Healer

{T}: Prevent the next 3 damage that would be dealt to any target this turn.

Argivian Phalanx

This spell costs {1} less to cast for each creature you control.
Vigilance

Sarpadian Empires, Vol. VII

As Sarpadian Empires, Vol. VII enters the battlefield, choose white Citizen, blue Camarid, black Thrull, red Goblin, or green Saproling.
{3}, {T}: Create a 1/1 creature token of the chosen color and type.

The Mirari Conjecture

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I — Return target instant card from your graveyard to your hand.
II — Return target sorcery card from your graveyard to your hand.
III — Until end of turn, whenever you cast an instant or sorcery spell, copy it. You may choose new targets for the copy.

Resolute Reinforcements

Flash
When Resolute Reinforcements enters the battlefield, create a 1/1 white Soldier creature token.

Augur of Bolas

When Augur of Bolas enters the battlefield, look at the top three cards of your library. You may reveal an instant or sorcery card from among them and put it into your hand. Put the rest on the bottom of your library in any order.

Sword of the Chosen

{T}: Target legendary creature gets +2/+2 until end of turn.

Historian's Boon

Whenever Historian's Boon or another nontoken enchantment enters the battlefield under your control, create a 1/1 white Soldier creature token.
Whenever the final chapter ability of a Saga you control triggers, create a 4/4 white Angel creature token with flying and vigilance.

Loran, Disciple of History

Whenever Loran, Disciple of History or another legendary creature enters the battlefield under your control, return target artifact card from your graveyard to your hand.

Shipwreck Dowser

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
When Shipwreck Dowser enters the battlefield, return target instant or sorcery card from your graveyard to your hand.

Glacial Floodplain

({T}: Add {W} or {U}.)
Glacial Floodplain enters the battlefield tapped.

Glacial Fortress

Glacial Fortress enters the battlefield tapped unless you control a Plains or an Island.
{T}: Add {W} or {U}.