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.'"

Crash

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

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."

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.

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, put two +1/+1 counters on target creature. If that's another creature, it gains the following ability until end of turn.)
Trample

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.)

Pulverize

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

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}.

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.

Landslide

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

Fireblast

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

Granite Grip

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

Fire Dragon

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

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.

Staff of the Flame Magus

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

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.

Lithophage

At the beginning of your upkeep, sacrifice Lithophage unless you sacrifice a Mountain.

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.

Seismic Strike

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

Dwarven Pony

{1}{R}, {T}: Target Dwarf creature gains mountainwalk until end of turn. (It can't be blocked as long as defending player controls a Mountain.)

Oxidda Golem

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

Glacial Crevasses

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

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.

Spitting Earth

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

Anger

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

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'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.

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.

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.

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.

Akki Raider

Whenever a land is put into a graveyard from the battlefield, Akki Raider gets +1/+0 until end of turn.

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.

Dwarven Grunt

Mountainwalk (This creature can't be blocked as long as defending player controls a Mountain.)

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.)

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.)

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.

Claws of Valakut

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