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.

Dwarven Mine

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

Skirk Volcanist

Morph—Sacrifice two Mountains. (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)
When Skirk Volcanist is turned face up, it deals 3 damage divided as you choose among one, two, or three target creatures.

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.

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.)

Claws of Valakut

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

Fire Dragon

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

Fireblast

You may sacrifice two Mountains rather than pay this spell's mana cost.
Fireblast deals 4 damage to any target.

Clifftop Retreat

Clifftop Retreat enters tapped unless you control a Mountain or a Plains.
{T}: Add {R} or {W}.

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.

Snow-Covered Mountain

Anger

Haste
As long as Anger is in your graveyard and you control a Mountain, creatures you control have haste.

The First Eruption

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I — The First Eruption deals 1 damage to each creature without flying.
II — Add {R}{R}.
III — Sacrifice a Mountain. If you do, The First Eruption deals 3 damage to each creature.

Furycalm Snarl

As Furycalm Snarl enters, you may reveal a Mountain or Plains card from your hand. If you don't, Furycalm Snarl enters tapped.
{T}: Add {R} or {W}.

Balduvian Trading Post

If Balduvian Trading Post would enter, sacrifice an untapped Mountain instead. If you do, put Balduvian Trading Post onto the battlefield. If you don't, put it into its owner's graveyard.
{T}: Add {C}{R}.
{1}, {T}: Balduvian Trading Post deals 1 damage to target attacking creature.

Spawn of Thraxes

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

Rockslide Ambush

Rockslide Ambush deals damage to target creature equal to the number of Mountains you control.

Seismic Strike

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

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.

Ben-Ben, Akki Hermit

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

Spitting Earth

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

Pulverize

You may sacrifice two Mountains rather than pay this spell's mana cost.
Destroy all artifacts.

Goblin Ski Patrol

{1}{R}: Goblin Ski Patrol gets +2/+0 and gains flying. Its controller sacrifices it at the beginning of the next end step. Activate only once and only if you control a snow Mountain.

Valakut, the Molten Pinnacle

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

Sol Ring

Kyren Legate

If an opponent controls a Plains and you control a Mountain, you may cast this spell without paying its mana cost.
Haste

Themberchaud

Trample
When Themberchaud enters, 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.)

Gauntlet of Might

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

Glacial Crevasses

Sacrifice a snow Mountain: Prevent all combat damage that would be dealt this turn.

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.

Akoum Hellkite

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

Chandra, Bold Pyromancer

+1: Add {R}{R}. Chandra, Bold Pyromancer deals 2 damage to target player.
−3: Chandra, Bold Pyromancer deals 3 damage to target creature or planeswalker.
−7: Chandra, Bold Pyromancer deals 10 damage to target player and each creature and planeswalker they control.

Blood Moon

Peak Eruption

Destroy target Mountain. Peak Eruption deals 3 damage to that land's controller.

Quintorius, Loremaster

Vigilance
At the beginning of your end step, exile target noncreature, nonland card from your graveyard. Create a 3/2 red and white Spirit creature token.
{1}{R}{W}, {T}, Sacrifice a Spirit: Choose target card exiled with Quintorius. You may cast that card this turn without paying its mana cost. If that spell would be put into a graveyard, put it on the bottom of its owner's library instead.

Arena of Glory

Arena of Glory enters tapped unless you control a Mountain.
{T}: Add {R}.
{R}, {T}, Exert Arena of Glory: Add {R}{R}. If that mana is spent on a creature spell, it gains haste until end of turn. (An exerted permanent won't untap during your next untap step.)

Torrent of Stone

Torrent of Stone deals 4 damage to target creature.
Splice onto Arcane—Sacrifice two Mountains. (As you cast an Arcane spell, you may reveal this card from your hand and pay its splice cost. If you do, add this card's effects to that spell.)

Dormant Volcano

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

Oxidda Golem

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

Simian Spirit Guide

Exile Simian Spirit Guide from your hand: Add {R}.

Chandra's Regulator

Whenever you activate a loyalty ability of a Chandra planeswalker, you may pay {1}. If you do, copy that ability. You may choose new targets for the copy.
{1}, {T}, Discard a Mountain card or a red card: 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."

Mogg Alarm

You may sacrifice two Mountains rather than pay this spell's mana cost.
Create two 1/1 red Goblin creature tokens.

Goblin Charbelcher

{3}, {T}: Reveal cards from the top of your library until you reveal a land card. Goblin Charbelcher deals damage equal to the number of nonland cards revealed this way to any target. If the revealed land card was a Mountain, Goblin Charbelcher deals double that damage instead. Put the revealed cards on the bottom of your library in any order.

Crash

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

Duergar Hedge-Mage

When Duergar Hedge-Mage enters, if you control two or more Mountains, you may destroy target artifact.
When Duergar Hedge-Mage enters, if you control two or more Plains, you may destroy target enchantment.

Awaken the Ancient

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

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.

Spire Barrage

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

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.

Chained to the Rocks

Enchant Mountain you control
When Chained to the Rocks enters, exile target creature an opponent controls until Chained to the Rocks leaves the battlefield. (That creature returns under its owner's control.)

Staff of the Flame Magus

Whenever you cast a red spell or a Mountain you control enters, you gain 1 life.

Ruby Medallion

Red spells you cast cost {1} less to cast.

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.

Ib Halfheart, Goblin Tactician

Whenever another Goblin you control becomes blocked, sacrifice it. If you do, it deals 4 damage to each creature blocking it.
Sacrifice two Mountains: Create two 1/1 red Goblin creature tokens.

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 you control enters, this emblem deals 4 damage to any target."

Thought Vessel

You have no maximum hand size.
{T}: Add {C}.

Alpine Guide

When Alpine Guide enters, 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.

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.'"

Lava Dart

Lava Dart deals 1 damage to any target.
Flashback—Sacrifice a Mountain. (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Wand of the Elements

{T}, Sacrifice an Island: Create a 2/2 blue Elemental creature token with flying.
{T}, Sacrifice a Mountain: Create a 3/3 red Elemental creature token.

Chandra, Dressed to Kill

+1: Add {R}. Chandra, Dressed to Kill deals 1 damage to up to one target player or planeswalker.
+1: Exile the top card of your library. If it's red, you may cast it this turn.
−7: Exile the top five cards of your library. You may cast red spells from among them this turn. You get an emblem with "Whenever you cast a red spell, this emblem deals X damage to any target, where X is the amount of mana spent to cast that spell."

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.

Granite Grip

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

Thunderclap

You may sacrifice a Mountain rather than pay this spell's mana cost.
Thunderclap deals 3 damage to target creature.

Earth Servant

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

Castle Embereth

Castle Embereth enters tapped unless you control a Mountain.
{T}: Add {R}.
{1}{R}{R}, {T}: Creatures you control get +1/+0 until end of turn.

Chandra, Novice Pyromancer

+1: Elementals you control get +2/+0 until end of turn.
−1: Add {R}{R}.
−2: Chandra, Novice Pyromancer deals 2 damage to any target.

Pearl Medallion

White spells you cast cost {1} less to cast.

Arid Mesa

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