Oxidda Golem

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

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}.

Disintegrate

Disintegrate deals X damage to any target. If it's a creature, it can't be regenerated this turn, and if it would die this turn, exile it instead.

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.

Mountain

Claws of Valakut

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

Spire Barrage

Spire Barrage deals damage to any target equal to 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.

Slith Firewalker

Haste
Whenever Slith Firewalker deals combat damage to a player, put a +1/+1 counter on it.

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.'"

Spawn of Thraxes

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

Spitting Earth

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

Embodiment of Fury

Trample
Land creatures you control have trample.
Landfall — Whenever a land you control enters, you may have target land you control become a 3/3 Elemental creature with haste until end of turn. It's still a land.

Seismic Strike

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