Howdy Lordnightshade2!
Unless you intended for this, its breath weapon is far far too weak. An average of 44 dmg in a 90 foot cone or 88 damage on a single target is nothing. By level 20 the pcs will have resistance at least against fire since its the most common resistance and 22 or 44 dmg against even a wizard is something they can easily shrug off. Heck even the normal ancient reds 91 dmg is reduced to 45 by pcs at this level and thats barely a threat considering the breath weapon is the main weapon of 5e dragons and can only be used once every 5 rounds on average. And at least with that attack it can hit all the PCs potential while focus fire not only does less damage but also targets one person, because why on earth would the pcs stack on top of on another.
Okay lets break this down.
1. The breath weapon can be used in conjunction with its bite and claws. Its no longer an either/or scenario.
2. Once bloodied, the dragons breath weapon jumps to 80/160* damage.
3. Insidious Gaze (Legendary Ability) let's the dragon strip away Fire Resistance.
4. Note this Ancient Red Dragon rates in at CR 20 (and its balanced for CR 20...even though it does far more damage than the official dragon stats).
*160 is higher than a maximised 26d6 from the official Ancient Red dragon.
Also a dragon should if it can help it always be flying, since its their main defensive strategy to avoid the bulk of dmg something like a barbarian or martial specialised fighter should do so its weak spot is extremely dangerous for the dragon since ranged attacks should be the main attacks against a dragon anyway.
Note how many of the dragon's attacks knock a target prone, grapple and throw them or swat them away (if problematic). Plus Smoke Snort can give a temporary Disadvantage debuff.
Flight is certainly an advantage, but increasingly less of an advantage the higher level the PCs are since they will likely also have flight or at least access to it.
The Dragon has Damage Threshold 20, so it needed a weak spot somewhere.
Legendary actions should be buffed. Reduce the tail cost to 1 since that way it can at least make their full attack and 2 tails strikes and because the tail strike is atleast in my view far less impt than the other two legendary actions it has. Like why on earth would it ever choose an attack over reducing giving a foe disadavantage on their attack and making a foe more suspectible for their breath weapon. At least with a tail as one it can buff its dmg output and debuff its opponents in one round.
I'll consider it. I have a specific way of balancing various attacks/abilities as Legendary Actions for monsters. I do admit they were initially set up to have 3 Legendary Actions (the reason its dropped to 2 is because of the CR drop, with Legendary Actions increasing by 1 per 4 CR after 13 - I'll think about buffing it to CR 21).
Flight actions are good and the bonus actions allow the dragon to choose between frightful or bonus damage so thats good since it can allow a dragon whose foes have succeeded the save to be able to buff their attack.
Thanks.
Also if this is a champion of a god shouldnt it have some trait to show that the god has given them its blessing.
That's a good idea.
In my defense, this monster is something of an after-thought, rather than a true champion. That said, it does guard the door to his workshop, so by rights it should have something unique to it...maybe a breastplate that covers its weak-spot (until some Rogue cuts the straps).
Its current skillset seems like something any ancient red could have.
True and technically that was my goal, simply to see what my take on an Ancient Red Dragon would be.
Maybe give it some spell casting or heck have it equip some magic armour hephaetus made for it or have its claws be enchanted since its claws and tail and what not appear to lack the magical weapons trait. Which admitted dragons dont have but it is suppose to be a champion of a god so it should be far stronger than a typical ancient red.
All monsters with Legendary Actions treated as if 'magical' for the purposes of attacks - I'm trying to keep the stat-blocks to 1 page (where possible, hence that wasn't mentioned).
Ardalus is the adopted Son of Hephaestus, I agree it should have some unique item/gift to set it beyond a typical Ancient Red (albeit I had only designed it as an Ancient Red to contrast my methods with the official counterpart).
Thank you very much for the feedback. Greatly appreciated.