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

[Islayre d'Argolh]

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

I don't think so...
It's mostly a question of aesthetics : i like the idea that some foes are powerful enough to be able fight alone.

For some of the "Figures/Alpha predators" of my campaign setting i will happily use minions [I'm considering a Winter Eladrin Warlord - CR 12 - with a pair of Winter wolves as hounds] but the Great Red Dragon is a solitary force of nature, an army all by itself.

 

[Paul Farquhar]

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

You can always have the cavalry come over the hill...

 

[J-H]

I agree that a base CR 17 dragon is not enough for a level 11 party.
At level 11, a Barbarian/Fighter with a few good rolls can do over 100 damage per round with GWM/Action Surge. So can a Paladin with a crit. Most other striker-types can do 40-60 with good rolls. If your dragon only has 300hp, it's down in 1-2 turns as long as the party can get to and stay in melee with it.

 

[aco175]

Something to consider as well is the power bump at 11th level. It is like the bump at 5th level where all of a sudden you find the PCs can handle a lot more. It sounds like a cool encounter with enough danger. I like having minions with solo creatures and may make a CR15 with a few CR7-9 monsters as allies to the BBEG, but this is why solos have lair actions and legendary actions.

 

[Quickleaf]

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.



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



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

A few powerful monsters I ran for my gang of experienced players...

Titanoboa (CR 12, Tome of Beasts, lots of dazzle/swallow effects, non-legendary) vs. four 5th-level PCs (pretty fresh) = pushed them to their limit, nearly killed two PCs, lucky rolls and clever tactics won the day, but they were totally depleted of resources at the end.

Star Spawn Larva Mage (CR 16, but homebrew lair actions & magic item & scene probably kicked it up to 17, MToF, non-legendary) vs. four 10th-level PCs (at ~2/3rds freshness) = very difficult, with some close calls, but PCs were able to win without losing anyone, albeit at the cost of many resources, and aching for a rest.

 

[Stalker0]

I think you can forego minions but should then focus on the terrain (aka volcano lair).

single monsters can really struggle against a whole party, and terrain can help smooth things out

 

[J-H]

Two variants I suggest:
Bite and Throw
On a successful bite, the dragon may choose to try to grab its target, with a roll of d20+(Dragon PB+Str) opposed to the target’s Athletics or Acrobatics roll. On a successful roll, the dragon will grab its prey, shake it back and forth, and then open its jaws, flinging the target into the air. The target will travel in a random direction in a straight line (roll 1d8, with 1 as north, circling around to 8 for NW) for 1d6 x 10 feet, stopping early if it hits an obstacle. The thrown target will take 1d6 falling damage per 10’ traveled (rounding down), Acrobatics DC 15 half. If the thrown creature hits another creature, both of them take that amount of falling damage, although the target may make a dexterity save (DC 15) to halve the damage.

Change up the Legendary Actions
Consider changing up the Legendary actions to give it good options for non-melee attacks. "Move" "Tail Slap" and "Look around" are all pretty boring, and it may not want to be in melee with the party or particular party members. I like to add "Cast a spell" at a cost of 2 actions. Crown of Stars is a nice one because it gives the dragon a bonus action ranged attack. Heal is another good choice. It only heals 70hp (1-2 player turns of damage), but it clears off some debuffs, and sends a message to the players that they shouldn't hold back.

If you want to really make things hard, give it Improved Invisibility. Mwa ha ha ha ha!

 

[Quartz]

If you want to really make things hard, give it Improved Invisibility. Mwa ha ha ha ha!

Nasty!

 

[Islayre d'Argolh]

I agree that a base CR 17 dragon is not enough for a level 11 party.

And with the spells and a few HP more (let's say 30-50 bonus HP, a very good roll with the 19d12 ^^) ?

 

[Islayre d'Argolh]

Star Spawn Larva Mage (CR 16, but homebrew lair actions & magic item & scene probably kicked it up to 17, MToF, non-legendary) vs. four 10th-level PCs (at ~2/3rds freshness) = very difficult, with some close calls, but PCs were able to win without losing anyone, albeit at the cost of many resources, and aching for a rest.

I'm seriously considering this one, reskinned as the King in Yellow.
I could use it as a potential Patron for a Great Old One Sorcerer...