Let the Dragon use Magic items as it is a freaking Ancient Red Dragon. It likely has quite a few magic items in its hoard. Also have Glyphs of Warding so it can fly to throughout the lair and use them to buff/heal itself with. Think Beacon of Hope, it maximizes all healing done to a creature. Have a few Glyphs with Healing Spirit in them in an out of sight position he can fly to. Have potions stashed in strategic locations the dragon can fly to. You can make the fight really tough without changing anything on the dragons base stats from the book.
D&D 5E Setting Party level vs an Ancient Red Dragon
Oofta
Legend
The simple solution is to look at the DMG guidelines page 274 creating a monster and add AC, HP, etc based on the current CR vs the target CR.
So in your case current CR is 17, you are targeting 21. Prof bonus increases by 1, AC doesn't change, HP increases by 90, etc. Most of it's pretty straightforward, although increasing damage can take a little work. I've also added class levels, but figuring out the CR is not as simple (I don't usually pay too much attention to that).
As far as lair actions, if you think it would add to the enjoyment of the game and makes sense I'd add them. They aren't going to make that much of a difference.
Good luck and let us know how it goes!
One player just reminded me of something important. Due to some strange shenanigans that went on previously involving an ancient magitech analog to the Deck of Many Things, a PC has 3 wishes up his sleeve. Note that the Dragon knows about this because a former party member is in the Dragon's thrall.
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hawkeyefan
Legend
[MENTION=467]Reynard[/MENTION]
As a rule of thumb, I'd add +2 to Hit and Damage rolls, and add a 1d6 to the fire damage from its attacks. I think the CR rules in the DMG would only indicate a +1 to attacks, but I'd go a little more.
Raise its AC to 21, which won't make a huge difference, but will turn a few hits into misses
Raise the DC for Frightful Presence, Breath Weapon, and Wing Attack by 1 (Wis DC 20, Dex DC 22, Dex DC 23, respectively).
I'd also add a good amount of HP...probably about 100. Or give it some other means to help mitigate damage. A simple Potion of Invulnerability would allow it to take half damage from all of the PC attacks for 10 rounds.
These increases put it somewhere between the CR 17 Adult and the CR 21 Ancient.
However, I think the key to this will be to increase the number of Legendary Actions and uses of Legendary Resistance. I think you could double each, at least. And as [MENTION=6785438]Warmaster Horus[/MENTION] suggested, maybe create a couple of unique Legendary Actions that you think would work well given the encounter design, terrain, party, etc.
Maybe a "Tail Sweep" that allows a tail attack against up to 3 adjacent enemies, that also forces a save or fall prone.
Maybe a "Snatch and Grab" that allows the Dragon to move up to half its movement without provoking, and make a grapple against one enemy. If grappled, the enemy is held in the dragon's claws and needs to succeed on a Dex-Acrobatics or Str-Athletics check to escape.
As a rule of thumb, I'd add +2 to Hit and Damage rolls, and add a 1d6 to the fire damage from its attacks. I think the CR rules in the DMG would only indicate a +1 to attacks, but I'd go a little more.
Raise its AC to 21, which won't make a huge difference, but will turn a few hits into misses
Raise the DC for Frightful Presence, Breath Weapon, and Wing Attack by 1 (Wis DC 20, Dex DC 22, Dex DC 23, respectively).
I'd also add a good amount of HP...probably about 100. Or give it some other means to help mitigate damage. A simple Potion of Invulnerability would allow it to take half damage from all of the PC attacks for 10 rounds.
These increases put it somewhere between the CR 17 Adult and the CR 21 Ancient.
However, I think the key to this will be to increase the number of Legendary Actions and uses of Legendary Resistance. I think you could double each, at least. And as [MENTION=6785438]Warmaster Horus[/MENTION] suggested, maybe create a couple of unique Legendary Actions that you think would work well given the encounter design, terrain, party, etc.
Maybe a "Tail Sweep" that allows a tail attack against up to 3 adjacent enemies, that also forces a save or fall prone.
Maybe a "Snatch and Grab" that allows the Dragon to move up to half its movement without provoking, and make a grapple against one enemy. If grappled, the enemy is held in the dragon's claws and needs to succeed on a Dex-Acrobatics or Str-Athletics check to escape.
Lidgar
Gongfarmer
Are you giving the dragon spellcasting abilities? I certainly would - and would be sure to include Counterspell and Dispel Magic in its spell list.
May not really counteract the Wishes, but I can't think of a good countermeasure for those...
Aww say it ain't so! I was really looking forward to hearing how well they could do against the big red.
Oh, but you did mention that you have only have 8 players now, so that makes sense. That's actually a pretty big loss in power.
I am going to model it on one of the slightly lesser CR metallic ancients. I like the idea of surprising them with an additional breath weapon.
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Oh, so with 3 wishes, that's a heck of an advantage.
For example, one wish even specified in the spell description is "complete immunity from one attack". So they could make themselves immune to the dragon's breath weapon for example.
Even just going based on 8th level spells or lower, again heroes feast for a massive buff and fear immunity. I'm sure there are some great spells to knock out a dragon...so definitely don't hold back.
I am not sure the math is right on that. CR21 should only be Hard for the party you described. KFC uses the DMG rules, and I double-checked it there as well. I would even go so far as to say the fight is only Medium. My encounter building spreadsheet smooths out the jumps when you add monsters or PCs at certain points so that each monster or PC adjusts the math gradually. While the DMG says that 8 PCs and above fighting 1 monster applies a .5 modifier to the total monster XP, I would suggest that 9 PCs actually makes the modifier .33.
http://www.enworld.org/forum/rpgdownloads.php?do=download&downloadid=1186
You can also use my monster CR calculator to plug in the Adult Red Dragon's specs and then modify them, add the lair action damage, etc. and see where the dragon ends up.
http://www.enworld.org/forum/rpgdownloads.php?do=download&downloadid=1189
Personally, I would enter the CR 17 dragon's specs (including damage from legendary actions), enter the lair action damage (remember you cannot use the same action consecutively), adjust the desired CR from 17 to 21 (should really be 22) to adjust the proficiency bonus, and see how close that gets you. After that, boost the hp until it gets the CR to what you desire. That will help keep you from a quick TPK that you might have caused by adding too much to damage.
What about using, say, the Ancient Bronze as a base instead? I could just reskin it Red.
Speaking of, it seems odd versatility does not appear to be included in CR calculations. That is, the metallic dragon's multiple breath options does not seem to have any impact on their relative CR.
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Speaking of, it seems odd versatility does not appear to be included in CR calculations. That is, the metallic dragon's multiple breath options does not seem to have any impact on their relative CR.
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