Claws of Valakut

Enchant creature
Enchanted creature gets +1/+0 for each Mountain you control and has first strike.

Barrel Down Sokenzan

Sweep — Return any number of Mountains you control to their owner's hand. Barrel Down Sokenzan deals damage to target creature equal to twice the number of Mountains returned this way.

Awaken the Ancient

Enchant Mountain
Enchanted Mountain is a 7/7 red Giant creature with haste. It's still a land.

Madblind Mountain

({T}: Add {R}.)
Madblind Mountain enters the battlefield tapped.
{R}, {T}: Shuffle your library. Activate only if you control two or more red permanents.

Navigation Orb

{2}, {T}, Sacrifice Navigation Orb: Search your library for up to two basic land cards and/or Gate cards, reveal those cards, put one onto the battlefield tapped and the other into your hand, then shuffle.

Brass's Bounty

For each land you control, create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")

Scaretiller

Whenever Scaretiller becomes tapped, choose one —
• You may put a land card from your hand onto the battlefield tapped.
• Return target land card from your graveyard to the battlefield tapped.

Seer's Sundial

Landfall — Whenever a land enters the battlefield under your control, you may pay {2}. If you do, draw a card.

Oliphaunt

Trample
Whenever Oliphaunt attacks, another target creature you control gets +2/+0 and gains trample until end of turn.
Mountaincycling {1} ({1}, Discard this card: Search your library for a Mountain card, reveal it, put it into your hand, then shuffle.)

Ghirapur Orrery

Each player may play an additional land on each of their turns.
At the beginning of each player's upkeep, if that player has no cards in hand, that player draws three cards.

Myriad Landscape

Myriad Landscape enters the battlefield tapped.
{T}: Add {C}.
{2}, {T}, Sacrifice Myriad Landscape: Search your library for up to two basic land cards that share a land type, put them onto the battlefield tapped, then shuffle.

Mountain

Staff of the Flame Magus

Whenever you cast a red spell or a Mountain enters the battlefield under your control, you gain 1 life.

Dormant Volcano

Dormant Volcano enters the battlefield tapped.
When Dormant Volcano enters the battlefield, sacrifice it unless you return an untapped Mountain you control to its owner's hand.
{T}: Add {C}{R}.

Crucible of Worlds

Snow-Covered Mountain

Cragsmasher Yeti

Mountaincycling {2} ({2}, Discard this card: Search your library for a Mountain card, reveal it, put it into your hand, then shuffle.)
Backup 2 (When this creature enters the battlefield, put two +1/+1 counters on target creature. If that's another creature, it gains the following ability until end of turn.)
Trample

Canoptek Wraith

Wraith Form — Canoptek Wraith can't be blocked.
Transdimensional Scout — When Canoptek Wraith deals combat damage to a player, you may pay {3} and sacrifice it. If you do, choose a land you control. Then search your library for up to two basic land cards which have the same name as the chosen land, put them onto the battlefield tapped, then shuffle.

Gauntlet of Might

Red creatures get +1/+1.
Whenever a Mountain is tapped for mana, its controller adds an additional {R}.

Terrain Generator

{T}: Add {C}.
{2}, {T}: You may put a basic land card from your hand onto the battlefield tapped.

Thespian's Stage

{T}: Add {C}.
{2}, {T}: Thespian's Stage becomes a copy of target land, except it has this ability.

Landslide

Sacrifice any number of Mountains. Landslide deals that much damage to target player or planeswalker.

Fabled Passage

{T}, Sacrifice Fabled Passage: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle. Then if you control four or more lands, untap that land.

Seismic Strike

Seismic Strike deals damage to target creature equal to the number of Mountains you control.

Sardian Cliffstomper

As long as it's your turn and you control four or more Mountains, Sardian Cliffstomper gets +X/+0, where X is the number of Mountains you control.

Firecat Blitz

Create X 1/1 red Elemental Cat creature tokens with haste. Exile them at the beginning of the next end step.
Flashback—{R}{R}, Sacrifice X Mountains. (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Walking Atlas

{T}: You may put a land card from your hand onto the battlefield.

Oxidda Golem

Affinity for Mountains (This spell costs {1} less to cast for each Mountain you control.)
Haste

Ben-Ben, Akki Hermit

{T}: Ben-Ben, Akki Hermit deals damage to target attacking creature equal to the number of untapped Mountains you control.

Mine Collapse

If it's your turn, you may sacrifice a Mountain rather than pay this spell's mana cost.
Mine Collapse deals 5 damage to target creature or planeswalker.

Koth, Fire of Resistance

+2: Search your library for a basic Mountain card, reveal it, put it into your hand, then shuffle.
−3: Koth, Fire of Resistance deals damage to target creature equal to the number of Mountains you control.
−7: You get an emblem with "Whenever a Mountain enters the battlefield under your control, this emblem deals 4 damage to any target."

Hithlain Rope

Hithlain Rope can't be sacrificed.
{1}, {T}: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle. The player to your right gains control of Hithlain Rope.
{2}, {T}: Draw a card. The player to your right gains control of Hithlain Rope.

Ruin Grinder

Menace
When Ruin Grinder dies, each player may discard their hand and draw seven cards.
Mountaincycling {2} ({2}, Discard this card: Search your library for a Mountain card, reveal it, put it into your hand, then shuffle.)

Downhill Charge

You may sacrifice a Mountain rather than pay this spell's mana cost.
Target creature gets +X/+0 until end of turn, where X is the number of Mountains you control.

Genju of the Spires

Enchant Mountain
{2}: Enchanted Mountain becomes a 6/1 red Spirit creature until end of turn. It's still a land.
When enchanted Mountain is put into a graveyard, you may return Genju of the Spires from your graveyard to your hand.

Alpine Guide

When Alpine Guide enters the battlefield, you may search your library for a Mountain card, put that card onto the battlefield tapped, then shuffle.
Alpine Guide attacks each combat if able.
When Alpine Guide leaves the battlefield, sacrifice a Mountain.

Crash

You may sacrifice a Mountain rather than pay this spell's mana cost.
Destroy target artifact.

Sol Ring

Eternal Flame

Eternal Flame deals X damage to target opponent or planeswalker and half X damage, rounded up, to you, where X is the number of Mountains you control.

Earth Servant

Earth Servant gets +0/+1 for each Mountain you control.

Chartooth Cougar

{R}: Chartooth Cougar gets +1/+0 until end of turn.
Mountaincycling {2} ({2}, Discard this card: Search your library for a Mountain card, reveal it, put it into your hand, then shuffle.)

Jaws of Stone

Jaws of Stone deals X damage divided as you choose among any number of targets, where X is the number of Mountains you control as you cast this spell.

Wayfarer's Bauble

{2}, {T}, Sacrifice Wayfarer's Bauble: Search your library for a basic land card, put that card onto the battlefield tapped, then shuffle.

Mogg Salvage

If an opponent controls an Island and you control a Mountain, you may cast this spell without paying its mana cost.
Destroy target artifact.

Fiery Emancipation

If a source you control would deal damage to a permanent or player, it deals triple that damage to that permanent or player instead.

Cleansing Wildfire

Destroy target land. Its controller may search their library for a basic land card, put it onto the battlefield tapped, then shuffle.
Draw a card.

Spawn of Thraxes

Flying
When Spawn of Thraxes enters the battlefield, it deals damage to any target equal to the number of Mountains you control.

Geomancer's Gambit

Destroy target land. Its controller may search their library for a basic land card, put it onto the battlefield, then shuffle.
Draw a card.

There and Back Again

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I — Up to one target creature can't block for as long as you control There and Back Again. The Ring tempts you.
II — Search your library for a Mountain card, put it onto the battlefield, then shuffle.
III — Create Smaug, a legendary 6/6 red Dragon creature token with flying, haste, and "When this creature dies, create fourteen Treasure tokens."

Chaos Moon

At the beginning of each upkeep, count the number of permanents. If the number is odd, until end of turn, red creatures get +1/+1 and whenever a player taps a Mountain for mana, that player adds an additional {R}. If the number is even, until end of turn, red creatures get -1/-1 and if a player taps a Mountain for mana, that Mountain produces colorless mana instead of any other type.

Gamble

Search your library for a card, put that card into your hand, discard a card at random, then shuffle.

Spitting Earth

Spitting Earth deals damage to target creature equal to the number of Mountains you control.

Blood Moon

War Room

{T}: Add {C}.
{3}, {T}, Pay life equal to the number of colors in your commanders' color identity: Draw a card.

Koth of the Hammer

+1: Untap target Mountain. It becomes a 4/4 red Elemental creature until end of turn. It's still a land.
−2: Add {R} for each Mountain you control.
−5: You get an emblem with "Mountains you control have '{T}: This land deals 1 damage to any target.'"

Themberchaud

Trample
When Themberchaud enters the battlefield, he deals X damage to each other creature without flying and each player, where X is the number of Mountains you control.
You may exert Themberchaud as he attacks. When you do, he gains flying until end of turn. (An exerted creature won't untap during your next untap step.)

Solemn Simulacrum

When Solemn Simulacrum enters the battlefield, you may search your library for a basic land card, put that card onto the battlefield tapped, then shuffle.
When Solemn Simulacrum dies, you may draw a card.

Akoum Hellkite

Flying
Landfall — Whenever a land enters the battlefield under your control, Akoum Hellkite deals 1 damage to any target. If that land is a Mountain, Akoum Hellkite deals 2 damage instead.

Buried Ruin

{T}: Add {C}.
{2}, {T}, Sacrifice Buried Ruin: Return target artifact card from your graveyard to your hand.

Horn of Greed

Whenever a player plays a land, that player draws a card.

Reckless Handling

Search your library for an artifact card, reveal it, put it into your hand, shuffle, then discard a card at random. If an artifact card was discarded this way, Reckless Handling deals 2 damage to each opponent.

Caged Sun

As Caged Sun enters the battlefield, choose a color.
Creatures you control of the chosen color get +1/+1.
Whenever a land's ability causes you to add one or more mana of the chosen color, add one additional mana of that color.

Reverberate

Copy target instant or sorcery spell. You may choose new targets for the copy.

Spire Barrage

Spire Barrage deals damage to any target equal to the number of Mountains you control.

Granite Grip

Enchant creature
Enchanted creature gets +1/+0 for each Mountain you control.

Fire Dragon

Flying
When Fire Dragon enters the battlefield, it deals damage to target creature equal to the number of Mountains you control.

Flameshot

You may discard a Mountain card rather than pay this spell's mana cost.
Flameshot deals 3 damage divided as you choose among one, two, or three target creatures.

Strata Scythe

Imprint — When Strata Scythe enters the battlefield, search your library for a land card, exile it, then shuffle.
Equipped creature gets +1/+1 for each land on the battlefield with the same name as the exiled card.
Equip {3}

Valakut, the Molten Pinnacle

Valakut, the Molten Pinnacle enters the battlefield tapped.
Whenever a Mountain enters the battlefield under your control, if you control at least five other Mountains, you may have Valakut, the Molten Pinnacle deal 3 damage to any target.
{T}: Add {R}.

Nesting Dragon

Flying
Landfall — Whenever a land enters the battlefield under your control, create a 0/2 red Dragon Egg creature token with defender and "When this creature dies, create a 2/2 red Dragon creature token with flying and '{R}: This creature gets +1/+0 until end of turn.'"

Burnished Hart

{3}, Sacrifice Burnished Hart: Search your library for up to two basic land cards, put them onto the battlefield tapped, then shuffle.

Siege of Towers

Replicate {1}{R} (When you cast this spell, copy it for each time you paid its replicate cost. You may choose new targets for the copies.)
Target Mountain becomes a 3/1 creature. It's still a land.

Demolition Field

{T}: Add {C}.
{2}, {T}, Sacrifice Demolition Field: Destroy target nonbasic land an opponent controls. That land's controller may search their library for a basic land card, put it onto the battlefield, then shuffle. You may search your library for a basic land card, put it onto the battlefield, then shuffle.

Valakut Stoneforge

Valakut Stoneforge enters the battlefield tapped.
{T}: Add {R}.

Valakut Awakening

Put any number of cards from your hand on the bottom of your library, then draw that many cards plus one.

Valakut Awakening // Valakut Stoneforge

Nahiri's Lithoforming

Sacrifice X lands. For each land sacrificed this way, draw a card. You may play X additional lands this turn. Lands you control enter the battlefield tapped this turn.

Price of Progress

Price of Progress deals damage to each player equal to twice the number of nonbasic lands that player controls.

Dwarven Mine

({T}: Add {R}.)
Dwarven Mine enters the battlefield tapped unless you control three or more other Mountains.
When Dwarven Mine enters the battlefield untapped, create a 1/1 red Dwarf creature token.