Reason for fighting: Archmage challenged them, they are all veryyyyyyy arrogant and accept.

Monsters: 1x Archmage 8x Cultists 1x Traag Draconian

Party level: 12

Special Circumstances: The 8 Cultists surround the archmage. Chanting intensely. They do not fight. Whilst all cultists live the Archmage is immune to all negative statuses and damage. When the cultists are killed the Archmage enters second phase and regains 2 spells of his choice and warp the party to a different place. (For thematic/cinematic reasons). When he inevitably is killed he will go into 3rd and last form transforming into a Traag Draconian with 40 temp hp. 

Does this sound ok?  

If the cultists stand close enough to the arch mage, a well placed fireball would take them all out. Maybe consider spreading them around the room or something instead of surrounding the arch mage?

Ah, yes. I have already accounted for that. Thank you though. I probably should've chosen better vocabulary.

And congrats on reaching 619 posts!

Cultists as pillars/keys - yep. 

Party Warp to new location - Yes. Consider this as a Lair Action at Initiative Count 20, or 10. Maybe both. Environment could be unlivable or terrain could be dangerous. 

Dropping to 0 HP and transformation - Also yes. But I am curious why the transformation would go from a CR 12 Archmage to a CR 5 Traag Draconian? Everything to this point has been an escalation, and at the very end there seems to be a fairly dramatic de-escalation. I was almost expecting a ratcheting up in challenge to something like a Star Spawn Seer. 

that depends. does it serve the story for the archmage to die? is this a 'proper' duel to the death? like, semi-formalized? if the archmage wanted to be clever he should stipulate that the contest ends when only one side remains standing or else all chanting has ceased (without revealing the role of the chanting in his defense). now if the heroes disable all the chanting acolytes they have ended the duel. but so too would a silence spell, Command: kneel!, or an earthquake where the archmage fails his dexterity save to not fall prone.

the one downside that nags me is that now the party would be much more unlikely to kill any of the cultists to brute force the encounter. but, hey, maybe they leave one guy standing and the archmage is sufficiently weakened. i think i'd have each of them pointing at the archmage with one hand and the cultist to their right with the right hand. pointing and chanting causes an obvious shield that would reduce spell effects such as fireball splash (and ranged attacks too?). it might take a perception check to see that, an insight check to realize it's aiding the archmage, and some good old fashioned trial 'n error to discover it doesn't block swords.

are any of the players beholden to a god of justice or order? perhaps the duel-pact was being observed by a third party? in this scenario flying into a rage and dragging the characters into a stage 2 would be a violation of the formalized duel. furthermore, an abundance of conjuration magic in the air might cause monodrone, duodrone, pentadrone, etc to begin accreting from thin air (well, from Mechanus) drawn by the chaos and breach of contract. if stage 3 becomes too much for the players, those constructs of the plane of order could come to their aide by clinging to the Draconian's feet and limiting their movement (and distracting them from any coup degrasse of downed player characters).

...and i just keep editing this longer and longer. sorry. and now i'm stealing it back. my heroes will be attempting to serve the archmage legal papers. he would not acknowledge messengers and lawyers so "adventurers" were sent to "investigate" tower of cultists. at the last he got wise and sought to turn the plane of Law against them, thus the formal duel. but now he's being dragged through a dimensional rift into Mechanus and the only way to stop it would be to accept the papers as a prior claim on his appearance in a court of law. neat!

but i'm also interested in hearing how your thing went too.

I can't see the creature from here. But update to the encounter. I've added a deep crow as the mages min. Would all this and the star seer still be beatable for a level 12 party?

Progressing from Cultists->Archmage->Star Spawn Seer gives you just a touch over the daily budget for a 4 member party @ 12LVL. Does that mean that this is beatable? Maybe, depending on dice rolls and tactics on both sides, it's possible. This encounter design isn't "all-in-at-once", it's effectively three encounters back-to-back with no short rests in between. Individually, they are beatable. After the party is worn down, this becomes dicey. (That's code for: No one can actually predict the outcome with any absolute precision. Unless they possess some type of supernatural ability to tell the future and read minds.)

Progressing from Cultists->Deep Crow & Traag Draconian->Archmage yields about the same benchmark for the daily budget, and therefore, a similar result. None of the individual encounters is above a Medium difficulty, but when run consecutively, YMMV.

I suppose the crux of my previous statement was centered on the de-escalation at the end. IMO, that would have been sorely anti-climatic and would likely serve the encounter better if it were omitted and the engagement were allowed to close cleanly with the defeat of the Archmage in Part 2.

Quote from Kaavel >>

 

I suppose the crux of my previous statement was centered on the de-escalation at the end. IMO, that would have been sorely anti-climatic and would likely serve the encounter better if it were omitted and the engagement were allowed to close cleanly with the defeat of the Archmage in Part 2.

 

Yes, and I see that now that you mentioned it. I have decided to run the encounter with the creatures i've already decided and I swapped out the Traag with the Star Spawn. The main reason I wen't with the Traag was I wanted it to be something similar to a resident evil moment when the mad scientist turns into this brunting brute.  De escalating it just a tad so the party isn't entirely overwhelmed.

the one downside that nags me is that now the party would be much more unlikely to kill any of the cultists to brute force the encounter. but, hey, maybe they leave one guy standing and the archmage is sufficiently weakened. i think i'd have each of them pointing at the archmage with one hand and the cultist to their right with the right hand. pointing and chanting causes an obvious shield that would reduce spell effects such as fireball splash (and ranged attacks too?). it might take a perception check to see that, an insight check to realize it's aiding the archmage, and some good old fashioned trial 'n error to discover it doesn't block swords.

 

This is already gonna be done, so no worries. The whole sheild thing is in play so the party thinks "Oh, nuke the boss like always and encounter done. When Its not that simple.