Combat-wise, it feels a bit much.
I recently started a druid. My first intent was to find a way to remain effective in combat despite depleted resources ; I found out that the druid hasn't much to offer, in that regard:
- Ranged options (such as sling attacks, or produce flame) lack
range and damage.
- Melee options (such as shillelagh or thorn whip) lack damage in the long run.
- AC is often on the low-end - unless your DM is generous on non-metal armor distribution, or you focus on dexterity.
It is indeed very frustrating, and encourages battlefield control over direct damage. Your homebrew subclass fixes all 3 issues, including strong blaster, healer and tank features: it would have been a no-brainer for me, and that's a warning for balance.
Bladesinger comparison
There are not that many full spellcasters with Extra Attack, and Bladesong looks very close to Heart of Fire. It may seem balanced... until you dig a little more:
\$\begin{array}{|c|c|c|}
\hline
\textbf{Ability} & \textbf{Bladesinger} & \textbf{Circle of flame}\\
\hline
\text{max AC using mundane gear} & 23 & \textbf{25} \\
\text{Requirements:} & \textit{mage armor}\text{ spell} & \text{shield, class level 18} \\
& \text{high dexterity & intelligence} & \text{high dexterity or nonmetal half-plate} \\
\hline
\text{max AC using magical items} & 25 & \textbf{31} \\
\text{Requirements:} & \textit{robe of the archmagi} & \text{+3 shield & armor, class level 18} \\
& \text{high Dexterity & Intelligence} & \text{high Dexterity or nonmetal half-plate} \\
\hline
\text{Hit dice} & \text{d6} & \textbf{d8} \\
\hline
\text{Damage control} & \text{reaction & spell slots-based} & \textbf{healing spells} \\
& \text{Damage reduction} & \textbf{reaction-based healing} \\
\hline
\text{Concentration}& \text{+Intelligence modifier} & \text{better AC = fewer concentration checks} \\
\hline
\end{array}
\$
All in all, the Circle of the Flame druid vastly outperforms its cousin - and can even do so with little ASI investment, should you find a proper armor.
Circle of Land & Circle of Spores comparisons
The Circle of Spores, from Unearthed Arcana, is probably an attempt to offer wannabe-melee druids an alternate to the circle of the moon's combat forms. Let's include it here:
\$\begin{array}{|c|c|c|}
\hline
\textbf{Subclass features} & \textbf{Circle of the Flame} & \textbf{Circle of the Land} & \textbf{Circle of Spores} \\
\hline
\text{level 2} & \textbf{$fire\ bolt$} & \text{one druid cantrip} & \textit{chill touch} \\
& \textbf{1-minute huge melee buff} & \text{Natural Recovery} & \text{10-min small melee buff} \\
& & & \text{reaction-based poison damage} \\
\hline
\text{level 6} & \textbf{Extra Attack} & \text{ignoring difficult terrain} & \text{1-hp zombies} \\
\hline
\text{level 10} & \textbf{+Wis mod to fire, concentration spells} & \text{various immunities} & \text{AoE spores} \\
& \textbf{fire resistance} & & \\
\hline
\text{level 14} & \textbf{turn fire/lightning damage into healing} & \text{beasts & plants hesitate to attack} & \text{various immunities} \\
\hline
\end{array}
\$
It looks like the Circle of the Flame has the best features at each level: best cantrip to allow ranged combat, best melee-buff, extra attack is sick, enhancing concentration-based spells is unique... Even the last feature, that appears situational at first, becomes cheesy once you start to fuel it with your own AoE spells.
Suggestions
So, how do we fix this?
First, you can't aim for balance, and be a strong healer, blaster and tank at the same time. I removed the healing capabilities, as the tank part seemed your main concern, and the theme calls for improved fire damage.
Second, on the AC question: "no metal armor" and bounded accuracy make things difficult.
If your DM rewards you with powerful nonmetal armors, he shouldn't allow big AC temporary bonuses in an homebrew class. The opposite is also true: if he allows big AC temporary bonuses... he should make sure you keep a lowly hide armor for your entire career. Hence the solution offered here - which seems the good way to reliably get good AC, without over-investing in dexterity or "forcing the hand" of your DM.
Here's the result:
Only You
When you choose the Circle of Flame at level 2, fire bolt and green-flame blade become druid cantrips for you. You learn an additional druid cantrip of your choice.
Leaves choices, and includes another fire-themed cantrip. Green-flame blade is mostly aimed at tier 2+ melee: extra attack was too much (especially if you intend to add Polearm Master), but this cantrip offers something comparable to the cleric's Divine Strike.
Heart of the Flame
At 2nd level, you gain the ability to awaken the primal spirits of flame. As a bonus action, you can expend a use of your Wild Shape feature to awaken your inner flame, rather than transforming into a beast form. While this feature is active, you gain the following benefits:
- You gain 3 temporary hit points per level you have in this class
- You shed bright light in a 30-foot radius and dim light for an additional 30 feet.
- You add your Wisdom modifier to concentration checks.
These benefits last for 10 minutes, or until you become unconscious.
Temporary HP is probably the way to go to add survivability to a druid: it matches what has been done for the Circle of Spores and Circle of the Moon. I removed the AC boost: if you insist on keeping it, you'll have to either keep it low (like +2), or remove the ability to use a shield - if you care about bounded accuracy, of course.
Purification through Fire
Starting at level 6, you gain resistance to fire damage. You also learn a purification ritual that allows you to prepare metal shields & armors, so that you can use them.
With your average 14-dex druid, it means up to +3 AC - and an easier access to magic armor in the long run. It is a game changer for most tables, where nonmetal armor is scarce - and can be balanced by making the ritual as simple or as complex as you wish (involving time, gold, crafting skills, a risk to destroy the item?).
Heat of Battle
Starting at level 10, once per turn, whenever one of your spells inflicts fire damage to a creature, you can add your Wisdom modifier to that damage.
Idea is to open that damage buff to more options (e.g fire bolt, green-flame blade) - while keeping it at a reasonable level (only one creature per turn).
Flaming Soul
Starting at level 14, you gain immunity to fire damage. You can also apply your heat of battle on a second damage roll, each turn.
Either affect both targets of green-flame blade, or use it on both flaming sphere and your cantrip of choice.
I'm absolutely not sure that this is well balanced:
- I feel Nature and/or Forge Domain clerics might feel cheated with this, as some of their core subclass features are included here, with few drawbacks
- having both a better AC and temporary HP, while keeping access to healing spells, means that you are tanking pretty hard
- Extra attack was too much - and this may also be. Green-flame blade + Heat of Battle + shillelagh makes a strong combination.
- Bonus to concentration might go out of control if you go for the War Caster / Resilient (Con) route. Replacing it with Con save proficiency or advantage on concentration checks may help avoid those shenanigans.
But I do think it is closer to "balanced", when compared with Land & Spores druids. Obviously, feel free to switch things around (access to metal armor as soon as level 2?) - and take or leave whatever fits best your idea.
This is almost balanced and ready for playtesting
In balance terms I think you have got this about as balanced as you can without playtesting, except for the 10th level feature. Fixing that then playtesting should be your next step. I expect you will find that some features are overpowered but the lack of slot recovery and the few underpowered feature balance it out overall. As usual I will still go through each feature and give some feedback.
Level 2
Bonus cantrip is good, you are giving access to something druids don't normally get so that makes up for that lack of choice compared to the Circle of the Land. I'm not fussed about the theme of Life/Death from Chaos but that is clearly what you are going for so that's alright. I would grant a more damaging cantrip like firebolt or acid splash to better suit the offensive theme.
1/20 chance of a free Wild Shape is cool and on theme. Though without the forms of Circle of the Moon and no casting in Wild Shape it isn't super useful. This feature is likely slightly underpowered compared to Natural Recovery but not too bad.
Circle Spells
- Crown of Madness: Yes! Totally on theme, a perfect choice
- Shatter: A good AoE attack spell that suits the theme. Good choice.
- Bestow Curse: Pretty good debuff spells that suits the theme. Not bad.
- Vampiric Touch: This is a little more death than Chaos to me. But I get that you want the healing from death type thing. Not a terrible spell choice.
- Death Ward: Not sure about the chaos part of this spell. It feels more like preventing chaos than causing it.
- Greater Invisibility: Not a bad option, it doesn't not suit the theme. Though you do have a lot of concentration spells to compete with it.
- Dawn: Was good before, still good now.
- Destructive Wave: Destructive chaos, basically built of this class. Good choice.
Most of your spell choices are pretty good. Some alternate suggestion for 7th level are: banishment, Evard's black tentacles, confusion, phantasmal killer and vitriolic sphere.
Also the suggestion by Rorp to give chaos bolt is awesome from a theme point of view. Unfortunately it is level 1 and doesn't suit the circle spell progression. I would consider finding a way to fit it in. Maybe instead of the free cantrip at 2nd level you give a 1/rest casting of Chaos Bolt.
Level 6
This is more balanced recharging only on a long rest. I'm not sold on the theme of it but it isn't an issue.
Level 10
Compared to the Circle of the Land this feature is way better. They get immunity to poison and disease, and charm and frightened from fey or elemental. Circle of the Moon get Elemental Wild Shape, Circle of the Shepherd get healing aura for summoned creatures, Circle of Dreams get a limited use teleport. You give advantage on all spell saves, all the time, with no restrictions.
As you say Yuan-ti Pureblood get this as a racial trait from 1st level. While this is support that the feature is not unbalance in general, it is still much stronger than anything the other circles get at this level.
The cantrip as a reaction thing is fine. It might be a little strong if you take my earlier suggestion to give a high damage cantrip but still not a huge deal. Combined with the other benefit is definitely pushes this into overpowered though. I would add some restriction to the magic resistance and cut this part for playtesting.
Level 14
Casting two spells in a turn 1/6 of the time could be really strong. However you will burn spells very quickly. I would limit it to not work on cantrips as that would just make it free damage with no cost. If this proves too strong during playtesting you could consider limiting this action to one weapon attack, Dash, Disengage, Hide, or Use an Object; the same as the action granted by the haste spell.
I love the theme of this ability and think you should stick with it, but be prepared to modify it if it proves too strong.
Why I think spell recovery is important
You say you want this class to be a primary caster druid. Every other primary offensive caster (Circle of the Land druid, wizard, sorcerer, warlock) have a way of regaining some spells on a short rest (sorcerer via sorcery points). Bard and cleric regain other magic-like abilities (Bardic Inspiration and Channel Divinity receptively) on a short rest. Circle of the Moon druids get their main offense (Wild Shape) back on short rests.
Most of these classes also have better offensive cantrips than the druid and don't rely so heavily on spell slots for damage. You are relying on spell slots and not giving a way to recover them. This puts you at a significant disadvantage compared to every other primary caster. That is why I have suggested it multiple times.
Best Answer
A fairly minor change
I think your take on the issues with the Circle of the Moon Druid is a fairly reasonable one. Having played one through to level 14, I agree there is a strong incentive to ensure your Wild Shape is available for combat as it is one of the strongest combat features in the game.
I'm going to address your features separately as I don't believe there is a strong reason for them to be tied together and players/GMs could easily use one or the other without consequences.
Native Forms
Given that you are granting a feature and taking nothing away this is inherently a power positive change, however I don't believe it is a bad one. A lot of the utility power of Wild Shape comes from being able to choose the best possible form for any given situation at a moments notice. By restricting to one (later two) forms you remove that ability and make this a fairly minor feature overall.
The benefit to roleplay and the "feel" of playing a Druid is far from minor. I, and every Druid player I've played with, would have loved this feature from a purely roleplay perspective. I believe you could even grant it earlier or raise the CR limit to 1 (but not both) without introducing a game breaking change. This feature doesn't really enable the Druid to do anything they couldn't do anyway but removes the need for taking a short rest immediately after, slowing down the game.
The potential for misuse is low due to the relatively weak forms and the potential for increased fun is high. That makes it a great change in my book and one I am likely to introduce in my own games.
Elder Druid
A Circle of the Moon Archdruid is brokenly powerful. It is by far the strongest capstone ability and can almost make them un-killable (par some instant kill effects). I believe the only reason it isn't a bigger issue is the rarity of level 20 play. For anyone that doesn't believe this watch Critical Role's Epic Level Battle Royal:
Therefore it makes sense that you might like to address this power imbalance. Given that you are taking away one of the strongest abilities in the game and replacing it with a weaker one this is obviously a power negative change. I'm going to ignore the part of Elder Druid that matches Archdruid and focus only on the first paragraph.
Regaining Wild Shape at the start of combat is undoubtedly useful and would likely lead to players using their wild shape ability more as there is no risk of being weaker for a combat. The is similar to what you may see with the original feature.
Being limited to two uses during combat is an enormous downgrade, the loss of hp, combat flexibility and endurance from the change is difficult to quantify. But I would estimate it at far far higher than 5hp per round.
The healing from your feature is so minor as to be basically meaningless at this level of play. Expected damage would be in the range 50-100 points per round rather than per minute so the healing won't make a meaningful difference to combat. It is also unnecessary between combats as you regain wild shape at the start of each combat and can always shift into a new form instantly regaining maximum hp.
I suggest scrapping the healing feature and instead granting 1 or 2 additional uses of Wild Shape. That should better mimic the feel and power of the original feature while removing the more broken aspects.
Conclusion
Overall these changes are relatively minor and unlikely to impact game balance in a meaningful way. Native Forms is a fun, very minor buff with few downsides. Elder Druid is a more significant de-buff but one far less likely to be relevant in most games.
I see no reason not to allow these at the table and encourage you to playtest and report back on if they resolve the issues you have encountered.