Hi,

 I’m running an encounter for my group of 7 5th level characters. They have been following this giant seed of an elemental tree trying to stop it. It’s kind of complicated, but  basically wherever it goes it leaves behind some form of elemental destruction. The latest is a volcano fueled by the magic of a volcano dragon (a home brew creation similar to an adult red dragon). The players have entered the volcano through a cave in attempts to escape the erupting lava and have found themselves trapped inside by a sheet of molten rock.

I am super hyped to run some kind of awesome fiery lava dungeon but I need some ideas to twist it up. I already have the standard stuff: hell hounds, mephits, fire elementals etc., but I want to get more interesting things on the mix that have to do with the environment. Obviously an encounter on a volcano is different than on the middle of an open field, but how can I put it to work. I want ideas that put pressure on the players outside of enemies, or cool lava related things that can spice up a fight if used properly. You get the point, I want it to be cool and memorable.

 Any suggestions?

Well, the first thing that comes to mind is that volcanic gasses are toxic.  There could be areas that have the functional equivalent of persistent Stinking Cloud or Cloudkill effects.  You could also have earth tremors that occur from time to time, which might force them to make saves to avoid being knocked prone or to dodge falling rocks, or open up or close paths.

An encounter on a giant lava pool, with some islands of cooled rock, so they need to jump from one island to the next. Though this will greatly favor ranged characters, so throw in some ranged enemies, too. And be careful of trying to push people into the lava. Maybe make the enemies fire/lava elementals, so the warlock with repelling blast doesn’t just start one-shotting everyone. Or use the Mephistopheles here do they’re flying anyway.

Falling stalactites.

Random geysers of steam or lava. I like to do something like, if you’re playing on a grid, have a player roll a d8 for direction, then a d6 for number of squares. Might do nothing,  might hit an enemy, might hit an ally. The random nature can be dangerous and make an encounter go tpk suddenly, so I’d keep the damage low. 

Or a hybrid of that and 6thlyranguard’s idea about clouds. Have the random effect spawn such a cloud in a 5 or 10 foot radius. 
I always love finding ways to reshape the battlefield on the fly. Too often it ends up with melee engaging and then is just static. Do making zone where you suddenly can’t stand anymore can really make things more dynamic.

How stable is it meant to be? Have the effects start low then ramp them up. According to a die roll (eg roll a d20, if it's above a certain threshold, things ramp up, below another threshold, it calms down, pick suitable numbers for how quickly you want to escalate it). When you reach peak, eruption occurs, everyone better drop what they're doing and flee!

Lava makes lava tubes, they usually have no stalagtites or stalagmites, unless they are very old and the volcano is dead.

They sometimes have lava underneath the lava tubes.  I.e. the lava burns through, then slowly subsides, creating a stream of lava, under a crust of cooled lava, under the empty air tube.

Quote from Mog_Dracov >>

Lava makes lava tubes, they usually have no stalagtites or stalagmites, unless they are very old and the volcano is dead.

Or if the magma intruded into an existing cave system.

If the magma intruded into an existing cave system then most likely the magma will destroy the ones on the ground at the very least, if not both.   Magma is heavy and strong and tends to spread out over the entire floor in cases like that.

Magma monsters?

Quote from Mog_Dracov >>

If the magma intruded into an existing cave system then most likely the magma will destroy the ones on the ground at the very least, if not both.   Magma is heavy and strong and tends to spread out over the entire floor in cases like that.

 In a fantasy world, you're not bound by such things and can arrange it based on whether or not you think it would be fun rather than whether or not it would be realistic. 

Night on Bald Mountain played on a loop.

The extreme heat may necessitate Con saves every x amount of time to prevent a level of exhaustion. As the PCs get closer to 'the source' the saves may become more frequent or the frequency stays the same but the difficulty increases.

Perhaps the extreme heat inhibits healing during short rests. Maybe the PCs needs to spend two dice for one die's worth of healing or there's a specific penalty to the die, like -2 or something. If a penalty and you're feeling generous maybe the hit points healed can't be less than one. :-)

Don't forget possible Lair Actions for your critter; magma explosions, gas jets, falling rocks, et al.

1) When the party enters an area near the center of the volcano, or an enclosure that is exposed to magma and steam, make anyone wearing metal armor roll a CON save due to conduction of the intense heat. On a fail, the character has disadvantage ability checks and attack rolls.  The condition ends after one minute after the character leaves the area. 

2) Have the players use a river of magma to travel to different areas. They can ride on large rocks the size of sea vessels (just without lower decks).  They will have to bring in their jumping abilities and athletics/acrobatics checks into the game.  Set some encounters while the players are riding along the rocks.  

3) Fissures resulting form tremors and quaking earth allow for creatures form the Underdark to emerge. 

4) Retool some creatures in the existing monsters manuals and reference books.  Modify stats and descriptions so they benefit from bine in a fiery environment. 

Remember that characters who have resistance to fire damage won't be affected by extreme heat that doesn't actually cause damage.