This spell is probably a bit more on the weaker side but damn, the flavour is amazing

Closer to a World Below plays in the background.

While it is a little bit weak, if you have a way to paralyze or stun a target it can do some impressive damage.

Quote from WillJewwel >>

While it is a little bit weak, if you have a way to paralyze or stun a target it can do some impressive damage.

Agree. Potentially effective against a paralyzed, stunned, petrified, or even restrained target. 

I have to disagree.  It takes two rounds, your concentration, a fifth level slot, and two failed saves to deal one target 12d6 points of damage of the most resisted damage types in the game.  This is doing 42 damage over two rounds - an average of 21 points of damage per round - and that's assuming two fails (other resources of players used to stun, etc.)  Sure, the second 4d6 doesn't take your action or a spell slot, but we're talking 1st or 2nd level spell damage there.  This is the absolute best case scenario - two failed Dex saves - and you're still only dealing 42 damage over two rounds.  Fireball, a level 3 spell, does an average of 28 over a 20 foot radius; upcast to 5th level, it does 35 damage as an area of effect - almost as much this spell does in two rounds while using your concentration.

By comparison, Animate Objects is a 5th level spell that takes your concentration, but you can use it to make 10 objects that have +8 to hit and deal an average of 6.5 points of damage each.  If only half of them hit, and with +8, this shouldn't be a huge issue, you'll do 32.5 points of damage in the first round.  You also have a 40% chance of a critical hit with ten attack rolls.  If you can somehow paralyze, stun, or otherwise impede a creature like suggested in the previous comments, your damage goes WAY up, with more hits and more crits.  While they can and will be destroyed (which may take away enemy actions), they can continue attacking using your bonus action - if they aren't attacked, they can do just as much damage on turn two as they did the first turn.  On top of all that, they can offer flanking if you use those rules, can make opportunity attacks, and have some utility outside of combat (ordering a door to stay shut, or serving as a flying platform for a bit, etc.)  Their big weakness is that they do non-magical damage, which is frequently resisted at higher tiers of play.  

Alternately, Synaptic Static does 8d6 damage at 5th level, the same as the first round of Immolation, and it targets a much better saving throw.  It's also area of effect, AND it offers a really great secondary benefit that persists, without requiring concentration.

This spell really needs to be improved to make it useful.  Remove the concentration requirement and up the damage and maybe you've got a good spell for times when you want to deal damage to a single target.  As it is, there's really no reason to ever prepare this, even if you get the spell for free.

I feel like the secondary damage should be moved to the start of the creatures turn, no save. Also the creature should have to spend an action to repeat the save to get rid of the effect and end the spell.

At the very least, this spell should use a CON save for the 4d6 damage-- if the target failed the DEX save initially, since it would already be on fire/burning.

Sorcerers will get a real kick out of twinning this spell.

Quote from Cryptobranchus >>

I have to disagree.  It takes two rounds, your concentration, a fifth level slot, and two failed saves to deal one target 12d6 points of damage of the most resisted damage types in the game.  This is doing 42 damage over two rounds - an average of 21 points of damage per round - and that's assuming two fails (other resources of players used to stun, etc.)  Sure, the second 4d6 doesn't take your action or a spell slot, but we're talking 1st or 2nd level spell damage there.  This is the absolute best case scenario - two failed Dex saves - and you're still only dealing 42 damage over two rounds.  Fireball, a level 3 spell, does an average of 28 over a 20 foot radius; upcast to 5th level, it does 35 damage as an area of effect - almost as much this spell does in two rounds while using your concentration.

By comparison, Animate Objects is a 5th level spell that takes your concentration, but you can use it to make 10 objects that have +8 to hit and deal an average of 6.5 points of damage each.  If only half of them hit, and with +8, this shouldn't be a huge issue, you'll do 32.5 points of damage in the first round.  You also have a 40% chance of a critical hit with ten attack rolls.  If you can somehow paralyze, stun, or otherwise impede a creature like suggested in the previous comments, your damage goes WAY up, with more hits and more crits.  While they can and will be destroyed (which may take away enemy actions), they can continue attacking using your bonus action - if they aren't attacked, they can do just as much damage on turn two as they did the first turn.  On top of all that, they can offer flanking if you use those rules, can make opportunity attacks, and have some utility outside of combat (ordering a door to stay shut, or serving as a flying platform for a bit, etc.)  Their big weakness is that they do non-magical damage, which is frequently resisted at higher tiers of play.  

Alternately, Synaptic Static does 8d6 damage at 5th level, the same as the first round of Immolation, and it targets a much better saving throw.  It's also area of effect, AND it offers a really great secondary benefit that persists, without requiring concentration.

This spell really needs to be improved to make it useful.  Remove the concentration requirement and up the damage and maybe you've got a good spell for times when you want to deal damage to a single target.  As it is, there's really no reason to ever prepare this, even if you get the spell for free.

 

 I have to disagree with you. This spell may be a bit weaker than other spells of its level, but with good teamwork and a simple Elemental Adept, you can deal ridiculous damage with this spell. Keep in mind that the 4d6 damage from this spell happens EVERY TIME they fail the DEX save after the initial hit. So it's 8d6, plus 4d6x10, which is a total of 48d6 damage, or on average with Elemental Adept, 192 damage over the course of 1 minute, at which point most combat encounters usually end. That is just too good to pass up, especially if you have a monk that can keep stunning the opponent, or something that can keep them restrained. 

adding to that, you also just need to make sure you cast the spell on a creature with low dexterity. 

I don't understand why this spell can't be extinguished by nonmagical means, but can end because the target succeeds on a Dexterity save. What are they doing as part of their save to extinguish the flames that no other nonmagical means can achieve?

I agree this is on the weak side, I think it'd be useable if the secondary damage occured before they made the saving throw, so if they fail the first save, they're guaranteed to take a further 4d6 damage. It's nice the followup damage requires no further action, I just think there's usually going to be a better way to use your 5th-level spell slots.

Good spell for homebrew dragon spell lists

It's a weak spell, though I can think of a couple of niche uses:

1. If it can't be extinguished by nonmagical means, that suggests it could be cast on a submerged target without the usual fire resistance. Perhaps useful for underwater campaigns.

2. If you are doing some intrigue, and you don't want a dead body or any remains, the ash-upon-death feature might be useful. Though Disintegrate is only one level up and much better.

I wish this was a Warlock spell. This is basically a high level full action variation of Hellish Rebuke. As someone playing a Fiend Warlock, it would be insanely good to have this spell for more round per round fire damage potentially.

agreed. 

If they fail the save they should be burning with magical fire. They take 4d6 at the start of their turn and must use an action to put out the fire. 

I agree with the comments regarding the weakness of this spell. However, there is one interaction that no one has mentioned. Consider the spell Darkness. It states the following (emphasis mine):

Magical darkness spreads from a point you choose within range to fill a 15-foot-radius sphere for the duration. The darkness spreads around corners. A creature with darkvision can't see through this darkness, and nonmagical light can't illuminate it.

If the point you choose is on an object you are holding or one that isn't being worn or carried, the darkness emanates from the object and moves with it. Completely covering the source of the darkness with an opaque object, such as a bowl or a helm, blocks the darkness.

If any of this spell's area overlaps with an area of light created by a spell of 2nd level or lower, the spell that created the light is dispelled.

So theoretically, since this spell causes a burning target to shed light in a 30-foot radius and since its 5th level, it would cause an area of magical darkness created by the Darkness to be dispelled. Granted, you would have to see the target and they would have standing close enough to an area of darkness AND fail their save, but this could be used to deal damage and dispel darkness. 

Still, the fact that it basically does as much damage as a Fireball and even then to just one single target makes this objectively worse. 

At the end of each of its turns, the target repeats the saving throw. It takes 4d6 fire damage on a failed save, and the spell ends on a successful one. this literally makes no sense because at the end of the spell description it literally says it can't be put out by non-magical means lol

I believe the succeeded save registers as "magical means" because the spell is what's causing there to be a save to succeed?

Like, the save is completely involuntary, so, maybe it doesn't narratively map to the effected "doing" anything other than just not being on fire anymore?

Even if it's not the best spell you have to admit the flavor of this spell is just so cool like I just imagine my character snapping their fingers like roy mustang and setting that one enemy on fire like a **** YOU specifically.

Would this work on a dead body in DnD?