So I'm starting with this dragon because I think a legendary CR 14 would often serve as the campaign boss for a common campaign (aka ones that end in your 8-10ish level range...which seems common based on WOTC polls). So that's my mindset, how does this dragon serve in that capacity? I did also look at the CR 18 to compare how the two scaled off each other.
Wisdom: I would bump up the wisdom to a 16. This rounds out the saving throws (no weaknesses!) and reinforces the "perceptive" dragon stereotype.
Spellcasting: I do like the straight up 5/day, pick what spells you want. A lot simpler and very inline with dragon's "sorcerer like casting". As noted in that other Enworld thread, spells are a pain for DMs to run, and many DMs find its the trickiest part of a creature to run. So I'm going to spend some real effort looking at the spells to see if they really make sense for the dragon. My goal is...what is the smallest array of spells that gets you the "Dragon experience" you are looking for.
Hellish Rebuke: I think its going to be very easy for a DM to miss this spell, they see a bunch of "action" spells and go "nah, I want my dragon to just get in melee and breathe fire and kick butt". However, as a reaction...this is theory should be a solid part of the Dragon's DPR (33 DPR, almost half the breath weapon). I guarantee that DMs are going to miss this effect. Further, I believe the desire was to make dragons less "spellcastery", while they have magics, they aren't necessarily spellcasters as a primary option. But with this spell, the dragon should be casting magic every round...which again feels off in flavor. Personally I would drop this spell and adjust the offensive DPR accordingly.
Flaming Sphere: Not a great spell to use in combat, but its alright if the dragon catches wind early and gets a round to prepare (gives them a bonus action option)...or uses it to send fire down a tunnel or something. So this spell has some use, I think its worth keeping it in.
Heal Metal: Unlike flaming sphere, you have to wait till combat stars to get use. While I love this spell, its still not a great trade off versus the dragon's normal tactics. However, I have an idea! (see my notes on Immolate Foe).
Fireball: Fireball to me is a good spell for the dragon. It mimics a mini breath weapon, gives the dragon some good range options...and if you hit enough people can actually compete on DPR with the dragon's normal attack routine. This feels like a spell worthy to use on the list.
Continual Flame: Pure fluff spell. I would just say in the lair description that a dragon often has continual lights throughout and be done with it.
Dragon Tactics: These need to be spelled out in the statblock. Right now you are expecting a lot of work from the DM, finding the abilities, scaling them based on age category, etc. Also something I noticed as I was looking over the tactics for this review, Snatch and Throw occurs much earlier in the block than Snatch....if I was a DM quickly building my monster....it would be very easy to see the first entry and assume they were the same (maybe it was a mistake and they just forgot the "and Throw" in the ability description). So I would also change the name of Snatch and Throw to something completely different.
Unstoppable: I personally like the previous version better (reaction to lose condition on your turn, or 30 damage + reaction to remove condition immediately). Problem with this version is, it leaves your dragon very vulnerable if it gets hit early in the round. A quick example, party blinds the dragon and entire group gets advantage on him and just wails into it, and the dragon doesn't get to deal with that until its turn. It leaves the dragon very "initiative vulnerable", which is something legendary monster stats specifically try to address.
I don't think there was anything wrong with the previous version. It functioned as a weaker legendary resistance, and did its job well.
Immolate Foe: I agree with other reviews that this ability is too niche to consume an action, legendary or otherwise. The 5e action economy is tight, if a creature is going to hold its weight (especially a "boss monster"), an action has to pressure the party. But I think it can be salvaged by merging it with the heat metal effect. So instead of having heat metal the spell, we include it as an effect from this ability. Such as:
Immolate Foe: The dragon selects up to one target it can see within 60 feet of it, which becomes wrapped in magical flames. The target must make a DC 18 Constitution saving throw or loose any resistance to fire damage for 1 hour. At the start of its turn, if the target is wearing metal armor or holding metal objects, it takes 5d8 fire damage and has disadvantage on saving throws and ability checks. This effect last for 1 minute.
What I've done is wrap most of the heat metal spell in with your resistance strip effect. Even though the damage effect is not that much compared to a normal dragon routine, by adding in an "automatic, mostly irresistible" damage to a single character, you suddenly put a lot of pressure on the party. This makes a dragon fight feel more than just "damage back and forth", but actually scary. Now this is an action worthy of replacing a dragon's full attack routine....at least some of the time.
Legendary Actions: This is a personal preference, but I prefer dragons with 3 legendary actions. My thinking is... legendary actions are to counter multiple party members, and its not like the party grows at higher levels. More legendary actions allows the dragon to adapt to the changing fight against multiple foes. Even if on paper a stronger dragon action + 2 legendary actions balances out....I think in play more LAs will create a more dynamic fight.
"Cool" Abilities: I feel that the CR 14 is missing some of the coolness you add in to the CR 18. The detect adding advantage is awesome (though its a bit unclear if it works on the next attack, or all attacks....I think it should on all attacks), I like how your ignite effect creates obstacles on the battleground and changes the terrain, and I feel the dragon is really missing out not having frightful presence. As I stated at the beginning of the review, I feel that CR 14 is where the first "campaign boss" type monsters should show up.... and so I would like see the CR 14 get some of these cool "boss like" effects. I completely agree that lower CR dragons don't need all the big stuff, they aren't alpha predators yet. But I think by CR 14, the dragon is ready to inherit the "big boss package"..... as this may be the highest CR dragon many parties will ever face.
