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.

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.

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.

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.

Granite Grip

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

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.'"

Fireblast

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

Spitting Earth

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

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.)

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.

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 enters the battlefield under your control, this emblem deals 4 damage to any target."

Claws of Valakut

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

Ben-Ben, Akki Hermit

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

Spire Barrage

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

Awaken the Ancient

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