alright taking my look at this bad boi. So the 2024 MM designs really tried to highlight the "less is more" concept. I see Dave has already streamlined a lot of things in this statblock, and I will continue to ask that question. What in the block is really adding meat to this dragon, versus things that seem kind of fun but if were removed really don't change the encounter.
Just want to echo my comment in my other response. I was just look at CR differences with this draft. It was not a redesign to follow the 2024 philosophy whole-hog. I have not decided how much I want to follow the 2024 design philosophy. There are things I like and things I don't.
Tail Slam / Tail Sweep: At the end of the day, I don't think these bring that much to the table. For the slam, the fire blast brings you more range than the 60 feet reach and has a superior condition. So you already have a good melee option in the bite and rend, and then the fire blast is an excellent long range option. I think the tail slam is superfulous. On the sweep, the Wing attack and the sweep really do a lot of the same things (wide area attack that does prone, and allows the dragon to distance themselves from a target with a high dex saving throw). Of the two, I think the wing attack is really all you need, great flexible AOE that gives you a prone and blind attack and then lets the dragon reposition as it needs to. The tail sweep has some niche moments, but if it was gone honestly wouldn't miss it a bit, the wing attack provides everything you need.
I disagree about the slam vs sweep, but agree there is a lot of overlap between the sweep and wing. I generally like more explicit options than the 2024 designs and giving the dragons options it should have. However, I will definitely review to see if there is a way to streamline and still provide what I want.
DCs: So we have DC 24s, 25s, and 26s in this statblock. One thing as a Dm, I HATE multiple spell DCs. hard for me to remember and a nightmare for my players to remember (hell even if a creature has a single spell DC I can't tell you how often I have to repeat the DC on every save....with multiple DCs there isn't a hope and a prayer my players will remember). I would just pick a lane and go with it.
I think your idea has merrit, but I also don't think it matters if you use a DC 25 when it should have been 26 instead. No need for a DM to remember, it doesn't matter as long as you are in the correct range. So this isn't a big deal to me one way or the other. There reason I haven't adopted this idea (you have suggested it before) is that I like the DC to mean something. If the dragon is stronger (DC 26) than it is magical (DC 24) I think the DC should reflect. It is a world building item as well as a monster building item. It is my simulationist side fighting against my gamist side.
Anti-Magic Field: So one rules question. Because the dragon attacks are "magical slashing damage"....do they work in an anti-magic field? I am sure the answer is supposed to be yes, but its a reasonable question for someone to ask.
I would leave that up to the DM. IMO, it would work the same as dragon flight. So if a dragon can fly in an anti-magic field, then its attacks do magical damage in an anti-magic field.
Legendary Actions: 2024 has made a concerted effort to get away from the "2 action/3 action" type legendary actions. I would ask if we can push that here? On the fire breath, I see the plan but I agree that its an odd design, goes against the new 2024 aesthetic, and also puts a lot of pressure on the very next pc after the charged breath, as they basically have to come up with some magic to stop the party from getting fried. So let me suggest a possible alternative:
This design predates (mostly) this new 2024 design philosophy. However, I am not as of yet convinced of the new LA philosophy. I understand it is easier, but I like the design flexibility of having multiple use LA. I will consider it though when I do a thorough review / redesign of the dragon.
Charge Breath: Now a legendary action, no longer a recharge rate (I liked your old dragon models where we got rid of the random chance much better)
Fire Breath: Now an action and as the clause (when used, the dragon cannot take any legendary actions until the start of its next turn). Would also love to get that damage into the 200ish range, because at that point you actually threaten instant death (aka double max hp) against some of the lower hp characters. If anything should be threatened the insta kill at CR27, its a dragon's breath weapon to me.
So that mostly mirrors what you are going for, but in a cleaner 2024 aesthetic (its not a perfect replication but I think it gets to the heart of what you are going for.
The issue with not having random or limited breath weapons is then why wouldn't the dragon just breath fire all the time. It makes the encounter less interesting / less dramatic IMO. Here my gamist side is fighting against my simulationist side (in reverse order of the DC issue). Also, if the CR is built around the dragon breathing fire every round, then the breath weapon damage needs to come down (not up to the 200is range you want). The only reason the breath weapon damage is high is because the claw/claw/bite option does much less damage.
I think you option here actively acts against a lot of what I am trying to do. I will consider it again, but I don't think this idea works for what I am trying to achieve.
Anti-magic field is the ultimate trump against many high level shenanagans. It allows the dragon to negate things like walls of force to corral it or getting tripped up by a lot of high level magical users. I think it would be a go to move for the dragon against many high level parties, and probably a "wish, negate a dragon's anti magic field" would be a classic move expected to go toe to toe with this beast.
What anti-magic field are you talking about?
The wall of fire can be surprisingly effective in the right terrain for its ability to block line of sight (though the dragon's height is a weakness here, it would need to be in places like tunnels where it can duck being the wall to truly prevent sighting. For the power word kill, I don't think it serves a big use here single it consumes the whole action and has a relatively short range. Its main use would be in taking out a creature immune to normal damage (which spells like wish or invulnerability could apply several types of invuls), so its a way to just knock those guys out, but you could remove it from the block with minimal fuss.
I think the real power though in the spell list is the forcecage. Simply put, this is a nigh guaranteed way to take a PC out of the fight every round. Even if they have teleportation, they still need to make a charisma saving throw (and against that dragons' DCs that is not happening for a lot of PCs). A smart dragon could easily knock out the rogue or like a barb for the fight.
So with that in mind, I would question the forcecage in the arsenal. It is clearly extremely useful...but I feel like it dominates the statblock a bit too much. The dragon has a number of other good conditions that are strong but the players can overcome them or just suffer them 1 round. The forcecage can just shut down a PC after pc for the entire fight with often few ways to get out of them, and turns this from a fight about the party beating an engine of destruction to them fighting a high level arch mage.
Good points. The forcecage thing is something I will think about. I don't mind the dragon having it, but I get your point. It is not something you want PCs toface a lot. However, this is an endgame level threat and I don't see the need to pull punches. Though it probably makes sense to move it to 1/day