Fireball

This spell costs {1} more to cast for each target beyond the first.
Fireball deals X damage divided evenly, rounded down, among any number of targets.

Desperate Ritual

Add {R}{R}{R}.
Splice onto Arcane {1}{R} (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.)

Volcanic Dragon

Smoke

Players can't untap more than one creature during their untap steps.

Lava Spike

Lava Spike deals 3 damage to target player or planeswalker.

Mountain

Meltdown

Destroy each artifact with mana value X or less.

Lava Blister

Destroy target nonbasic land unless its controller has Lava Blister deal 6 damage to them.

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}.

Volcanic Awakening

Destroy target land.
Storm (When you cast this spell, copy it for each spell cast before it this turn. You may choose new targets for the copies.)

Magmasaur

Magmasaur enters with five +1/+1 counters on it.
At the beginning of your upkeep, you may remove a +1/+1 counter from Magmasaur. If you don't, sacrifice Magmasaur and it deals damage equal to the number of +1/+1 counters on it to each creature without flying and each player.

Lava Runner

Haste
Whenever Lava Runner becomes the target of a spell or ability, that spell or ability's controller sacrifices a land of their choice.

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}.

Mudslide

Creatures without flying don't untap during their controllers' untap steps.
At the beginning of each player's upkeep, that player may choose any number of tapped creatures without flying they control and pay {2} for each creature chosen this way. If the player does, untap those creatures.

Wall of Lava

Defender (This creature can't attack.)
{R}: Wall of Lava gets +1/+1 until end of turn.

Incinerate

Incinerate deals 3 damage to any target. A creature dealt damage this way can't be regenerated this turn.

Lava Burst

Lava Burst deals X damage to any target. If Lava Burst would deal damage to a creature, that damage can't be prevented or dealt instead to another permanent or player.