D&D 5E Final Boss (tm) for a level 11 campaign.

[Islayre d'Argolh]

Well to be clear, the Legendary aspect doesn't, in theory, make the monster anymore challenging.

So you are asking if a CR 17 monster is a sufficient challenge for (4) lvl 11 PCs. In general my answer is yes, but it does depend on a lot of things. Some monsters are tougher for some groups than others. Some DMs are better at using monsters than others. Some players are better at tactics and group synergy than others. Without being able to account for all of these factors it is hard to say precisely.

From my own experience: I was able to barely TPK a group of (6) lvl 10 PCs with a CR 19 balor, so I think CR 17 monster for your group should be a good challenge.

Thanks
It's hard to believe that, in practice, Legendary actions & Legendary resistance do not make the monster more dangerous.
If we take a look at both the Goristro demon and the Adult Red dragon (CR17) they seems balanced before taking into consideration the Legendary features.

So, the thing to understand about CR is that it really only cares about how much damage the monster can deal in the number of turns it’s expected to be able to survive. The frame of reference the system is built around is one monster against 4 PCs of a level equal to its CR, dealing about a quarter of the party’s total HP in damage before it dies. Typically, that projection is based on its average damage over the course of 3 rounds, but some monsters deal higher damage and last fewer rounds, while others do less damage but last more rounds.

A CR 17 monster, then, can be expected to deal roughly a quarter of a 4-person, level 17 party’s health in the time it’s expected that such a party will take to kill that monster. Having Legendary Actions doesn’t typically affect its CR, unless those Legendary Actions allow it to survive more rounds or to deal more damage on average over the course of the number of rounds it’s expected to survive.

Precisely : Legendary actions include very frequently a 1-action-additional-attack of some sort. So the Legendary monster deal far more damage (once again, let's compare Goristro and Adult Red Dragon).
And Legendary restistance make the monster more durable

How many encounters have they had that day, and what magic items do they possess?

The truth is, i'm building a sandbox campaign setting and i want to know which CR & Legendary combination will be the more appropriate for the Figures/Alpha predators of the setting (i will not play past level 11 so i will have no use of Demon Princes, Kraken or Lichs for example).

Thnak you ALL for your good advices, a lot to think about

 

[Paul Farquhar]

Just a mention - the red dragon can do 56-ish points of damage on it's turn, but, taking it's legendary actions into account it can do another 51 with its tail attack in a round.

 

[Flamestrike]

The truth is, i'm building a sandbox campaign setting and i want to know which CR & Legendary combination will be the more appropriate for the Figures/Alpha predators of the setting (i will not play past level 11 so i will have no use of Demon Princes, Kraken or Lichs for example).

An 11th level party of 4 PC's (fully rested, no magic items) should find such an encounter a tough challenge, if the Dragon is played correctly and gets a terrain advantage.

Id make it a spellcasting variant (5 spells, max level 5, Save DC 19) dragon with the following spells: Mirror image, Shield, Scrying, Far Step, and Synaptic Static.

If given time (alerted by its minions of the PCs presence, which it almost certainly should be thanks to minions, Scrying or a familiar and Perception +13), it has Mirror Image, and Far Step pre-cast.

Far step gives you fantastic maneuverability and gives you something to do with your Bonus action each round. Bonus points is that it shuts down Walls of Force.

Hit them with Synaptic static (from outside counterspells 60' range via flying or far step teleporting) on round 1, targeting likely weak Int saves (dealing good damage, and inflicting a penalty of -1d6 on attack rolls against it). They wont expect psychic damage (fire damage for sure).

From there, use the breath weapon and judicial use of flying in, full attacking spellcasters, then teleporting back if needed to keep at range.

If your PC's have magic items, so should it (+1 ring of protection, Crystal ball (swap scrying for find familiar) and bracers of defense look pretty decent) for an AC of 22.

Ensure its minions have hallowed it's lair area, barring teleportation from non Dragons, and divination magic from same.

Environment should be something like the inside of a Volcano, plenty of room to fly, geysers of magma that act like Lair actions, pits of magma (and a lake of magma for it to escape into), and a narrow stone bridge, because why the hell not.

Include a Dragon Slayer (weapon your martial uses) in its hoard in plain sight, over the stone bridge. See if they're prepared to go after it. It automatically attunes to the first PC to attempt to wield it (it's sentient, hates the Dragon who spends most of its day trolling the poor sword, and grants advantage on attacks with itself against this particular Dragon).

Foreshadow the dragon, with minions hunting for the PCs as the advance in poer, rumors about the Dragon Slayer sword, and maybe even occasional sightings.

If you set it up right, it should be epic and a challenge even for experienced PCs.

 

[Islayre d'Argolh]

An 11th level party of 4 PC's (fully rested, no magic items) should find such an encounter a tough challenge, if the Dragon is played correctly and gets a terrain advantage.

Id make it a spellcasting variant (5 spells, max level 5, Save DC 19) dragon with the following spells: Mirror image, Shield, Scrying, Far Step, and Synaptic Static.

If given time (alerted by its minions of the PCs presence, which it almost certainly should be thanks to minions, Scrying or a familiar and Perception +13), it has Mirror Image, and Far Step pre-cast.

Far step gives you fantastic maneuverability and gives you something to do with your Bonus action each round. Bonus points is that it shuts down Walls of Force.

Hit them with Synaptic static (from outside counterspells 60' range via flying or far step teleporting) on round 1, targeting likely weak Int saves (dealing good damage, and inflicting a penalty of -1d6 on attack rolls against it). They wont expect psychic damage (fire damage for sure).

You're the second person suggesting the spellcasting variant, is that a indication that maybe legendary CR 17 is not strong enough for what i need ?

 

[Paul Farquhar]

You're the second person suggesting the spellcasting variant, is that a indication that maybe legendary CR 17 is not strong enough for what i need ?

Depends on the situation. If the players are rested, they take the dragon by surprise, it has no minions, and it can't get airborne it's probably toast in two rounds. I generally go significantly higher than "Deadly" for dragons (because dragons are supposed to be awesome) and for big bosses, but have something the players can use to swing the odds in their favour, as @Flamestrike suggests.

I wouldn't not use an ancient dragon against an 11th level party, but I would let them prepare by recruiting allies, buying potions of fire resistance, etc.

 

[dave2008]

Thanks
It's hard to believe that, in practice, Legendary actions & Legendary resistance do not make the monster more dangerous.
If we take a look at both the Goristro demon and the Adult Red dragon (CR17) they seems balanced before taking into consideration the Legendary features.

Legendary Actions and Legendary resistance figure into how you calculate the CR, but once the CR is calculated there is no difference. A CR 17 is, in theory, the same challenge as CR 17 Legendary. So if you remove a monsters LA & LR, they are most likely not the same challenge.

For example. Legendary resistance adds to the "effective" HP of the dragon. The red dragon actually has 346 "effective" HP (+90 HP for LA). And the Legendary Actions add 51 damage to its DPR (3 tail attacks).

Also, You will notice in the DMG encounter builder that it doesn't say anything about adjusting the encounter for legendary monsters.

FYI, per the DMG the red dragon CR is:
Defensive CR: 20
Offensive CR: 21
Total CR 20.5

So the dragon as listed is actual undervalued per the guidelines (this is a trait of the dragons in general). I think it is the same reason as the "fireball effect"

FYI, without the LR & LA the dragon is only CR 14.5

Now, that fact that LA give the monster more out of turn actions does in reality make it a bit more challenging, but not enough to change the CR.

 

[dave2008]

You're the second person suggesting the spellcasting variant, is that a indication that maybe legendary CR 17 is not strong enough for what i need ?

Probably not needed for a lvl 11 group, but it gives it more options and it depends on how prepared or skilled your group. In most cases a dragons natural attacks (either breath weapon or bite & 2 claws) are going to do more damage than any spell it could cast. It is more about giving the dragon more options if needed, depending on your party. This is not something that is figured into CR.

Personally I tend to go with themed spells (fire based of red dragons), but if you have a more savvy party, I would suggest these tactics (spells are at the end of the article): How to Play a Red Dragon like an Evil Genius

 

[Quartz]

Are there going to be minions? Powerful minions? A CR 17 dragon with powerful minions seems a bit too much for me.

Regardless I'd err on the side of making it look not so difficult on paper and have a couple of options for upscaling it in the fight if the fight is too easy for the players. An obvious ploy is the Mythic trait from Theros. Or maybe the dragon has a mature Clone stashed in the lava? "I was tired of that body anyway." Maybe there's more than one clone?

Remember that it's easier to scale an active encounter up than to scale it down.

 

[dave2008]

double post

 

[jgsugden]

You're the second person suggesting the spellcasting variant, is that a indication that maybe legendary CR 17 is not strong enough for what i need ?

I was the first to suggest it. For me, it is not about power, but about story. When I put the final battle of an adventure / campaign on the table, I want it to be one of the most memorable moments of the campaign. I want a battle map that blows them away with interesting and meaningful features. I want a figure for the monster that is terrifying. I want the players to feel a build up to the encounter that makes it feel like a climax (as opposed to just the next room in a dungeon), and I want them to be kept on their toes throughout the battle. I don't want them to ever feel like I pull my punches, but I don't want to put something in their path that they can't beat with the right decisions (although bad luck can also do them in when there is a tough encounter.

If I were giving it 5 spells, I would consider a combination that either sets up the dungeon as a challenge, or that entirely changes the battle midway through in a meaningful way, or disrupts the plans the PCs may have in place. Alternatively, they may be non-combat spells that just mess with the heads of the PCs.

Greater Invisibility.
Phantasmal Force.
Hallucinatory Terrain.
Summon (whatever)
Banishment (3 targets).
Globe of Invulnerability.
Armor of Agathys (6th level).
Bigby's Claw (Hand).
Dominate Person.
Gravity Sinkhole.
Erupting Earth.
Wall of Force.
Enemies Abound.
Antilife Shell.
Blindness/Deafness (5 targets)
Otiluke's Resilient Sphere.
Scrying.
Sending.
Contingency.
Dispel Magic.
Counter Spell.

I also have an insanely large list of homebrew spells from over the decades. Some are prepped for use in 5E, others are not - but I'll usually have between 25 and 50% of the spells of a spellcaster come from something homebrew. I recently had a new player join me at the table, and when the enemy spellcaster cast Deflection (as Counterspell, except before counterspelling it you make an arcana roll to identify the spell. If you do, and it has other options for targets, you can change the target of the spell and take control of it, being treated as the caster of it from there on - 6th level.), well - it changed everything.