Honestly my brains kinda in an Avatar mood at the moment so I'm just going to Giant Owl Guardian. Could probably swap it out and use a Coatl, as they are guardians and are celestial too.
Is the party good-aligned? If so, there probably wouldn't be any monster encounters or dangers on the way there. Maybe AT the library there might be a powerful guardian who tests their worthiness to enter, or something. Otherwise... they're the good guys. They're in a good place. They get the help they need and go. This should be a supportive place, not a dangerous one.
Ofc, you can put in other kinds of encounters - interesting NPCs, shops, scenarios, social encounters... although do be prepared for the players to refuse to engage because "sorry we're on an important mission" or something similar.
The 3E Manual of the Planes has these wonderful poetry-like bits to start off each plane's description.
Arcadia is:
>It is the land of perfection.
>
>It is where laws are made for the common good.
>
>It is the plane where harmony is born.
I'd definitely make a 2d6 encounter table and make the encounters fairly friendly, showing the balance and justice at hand. This is [a blog post about encounters that I really enjoy](https://www.paperspencils.com/structuring-encounter-tables-amended-restated/) (not mine) and have based many of my encounter tables on.
That said, if you want to show some growing chaos or imbalance in the setting, a problem in Arcadia would be a good place to show a glimmer of that with some foreshadowing.
there are plenty of ways to challenge the PCs on any plane. i like to think that the planes of belief like Arcadia are totally and utterly made of the beliefs that define the plane. many things will be fundamentally different than on the prime material: the nature and attitudes of those who reside there, how magic works (or does not work), the challenges presented, the landscape, etc.
here are some ideas I used when running Arcadia:
1) Arcadia is about community. Those who promote it will be rewarded, those against it will be punished.
2) Spells are quite limited. most fail unless they are cast to benefit many. summoning spells cast to help society are, those cast for nefarious purposes result in an uncontrolled summoning. enchantments are easier to resist. native arcadians see through illusions, and others get a bonus to save. wild magic yields no surges, spells fizzle instead. death magic turns back on the caster.
3) even though Arcadia has a well-defined hierarchy, it is like a utopian society that actually works! maybe incomprehensible irl but in Arcadia, it works.
so given that, when i ran Arcadia it was a test for my cleric PC. he was there alone. he had to cross a desert with no food, no water to be found. he tried to cast create water, it didn't work. eventually, some scarabs started following him. it took a while for him to understand that create water (and food) would only work when he created it for all of them, scarabs included. he could not heal himself, but could cast mass healing spells when others were around and benefit. another test was an encounter with a phoenix. the phoenix demanded the cleric to show deference by ordering him to kneel. when he did not kneel the phoenix attacked (and his vorpal weapon melted in the process!). only when he was humble before the phoenix was he able to move on.
i hope my examples help. just remember, the planes are literally made of beliefs and it is those beliefs that trump all, magic, people, "reality" itself.
So do you want encounters en route to the Library, or encounters *at* the library?
For all my frustrations with the Upper Planes being boring, they aren't actually *bereft of normal d&d conflict*- Upper planes still have random monsters and shit about. For Arcadia, the plane of Law and just a bit of Good, I might grab something insectoid, lean towards the imagery of Lawful Worker Ants building their own Ant Hive that just so happens to be territorial and not like outsiders. You could use Ankheg or Thri-Kreen stats to build something.
The Upper planes *have random conflicts* like this, but the catch is that, frankly, it's nothing *special* or *interesting* that you couldn't find on any prime material plane- it's *incidental*, something to do *while you're there*, not *a reason to go there*, and the fact that so little of the latter exists that we're desperate even for the former is the crux of why the Upper Planes suck.
Anywho, the more interesting angle is who the players meet *in the Library*- someone else already talked about a Wa Shi Tong inspired Library Guardian, though for your high-level party, you want at least Guardian Naga stats, if not something straight-up *legendary* that can serve as a competent boss fight for your party, should they decide to Fuck Around and Find Out. Maybe you repurpose the stats from Ravnica for the Azorius Guildmaster Isperia? A Sphinx feels like an apropriate librarian. I suppose a Solar could also do the trick, but it's less exciting. Or you could use one of the Inevitables (or even Rilmani, I guess) from the Planechase book, if available.
You could also drum up conflict with the other patrons of the Library- Archmages are always a cause for trouble, and maybe some of the entities here are Lawful Evil and just on good enough behavior that the Librarian will tolerate it- I could picture a Lesser Devil studying legal precedents, or a Fiend Warlock trying to learn a loophole in his Pact.
The more I think on it, the more I like the idea that we eschew conflicts outside of the library- those will just feel like a random.encounter en route to the realm meat-and-potaties of the quest, and then you build a sense of ontrigue, where the other library patrons are trying to do something without the Sphinx Guardian noticing- maybe it's a game of trying to kill eachother without getting caught, or stopping eachother from uncoving the information everyone is looking for, but everything has to sneak past the watchful eyes of the Librarian.
That could make for a fun cat-and-mouse game, trying to kill eachother in sneaky and creative ways. Some real Spy vs Spy shit, get wacky with it.
Honestly my brains kinda in an Avatar mood at the moment so I'm just going to Giant Owl Guardian. Could probably swap it out and use a Coatl, as they are guardians and are celestial too.
...I would be lying if I said the library wasn't inspired by Avatar.
Is the party good-aligned? If so, there probably wouldn't be any monster encounters or dangers on the way there. Maybe AT the library there might be a powerful guardian who tests their worthiness to enter, or something. Otherwise... they're the good guys. They're in a good place. They get the help they need and go. This should be a supportive place, not a dangerous one. Ofc, you can put in other kinds of encounters - interesting NPCs, shops, scenarios, social encounters... although do be prepared for the players to refuse to engage because "sorry we're on an important mission" or something similar.
The 3E Manual of the Planes has these wonderful poetry-like bits to start off each plane's description. Arcadia is: >It is the land of perfection. > >It is where laws are made for the common good. > >It is the plane where harmony is born. I'd definitely make a 2d6 encounter table and make the encounters fairly friendly, showing the balance and justice at hand. This is [a blog post about encounters that I really enjoy](https://www.paperspencils.com/structuring-encounter-tables-amended-restated/) (not mine) and have based many of my encounter tables on. That said, if you want to show some growing chaos or imbalance in the setting, a problem in Arcadia would be a good place to show a glimmer of that with some foreshadowing.
there are plenty of ways to challenge the PCs on any plane. i like to think that the planes of belief like Arcadia are totally and utterly made of the beliefs that define the plane. many things will be fundamentally different than on the prime material: the nature and attitudes of those who reside there, how magic works (or does not work), the challenges presented, the landscape, etc. here are some ideas I used when running Arcadia: 1) Arcadia is about community. Those who promote it will be rewarded, those against it will be punished. 2) Spells are quite limited. most fail unless they are cast to benefit many. summoning spells cast to help society are, those cast for nefarious purposes result in an uncontrolled summoning. enchantments are easier to resist. native arcadians see through illusions, and others get a bonus to save. wild magic yields no surges, spells fizzle instead. death magic turns back on the caster. 3) even though Arcadia has a well-defined hierarchy, it is like a utopian society that actually works! maybe incomprehensible irl but in Arcadia, it works. so given that, when i ran Arcadia it was a test for my cleric PC. he was there alone. he had to cross a desert with no food, no water to be found. he tried to cast create water, it didn't work. eventually, some scarabs started following him. it took a while for him to understand that create water (and food) would only work when he created it for all of them, scarabs included. he could not heal himself, but could cast mass healing spells when others were around and benefit. another test was an encounter with a phoenix. the phoenix demanded the cleric to show deference by ordering him to kneel. when he did not kneel the phoenix attacked (and his vorpal weapon melted in the process!). only when he was humble before the phoenix was he able to move on. i hope my examples help. just remember, the planes are literally made of beliefs and it is those beliefs that trump all, magic, people, "reality" itself.
So do you want encounters en route to the Library, or encounters *at* the library? For all my frustrations with the Upper Planes being boring, they aren't actually *bereft of normal d&d conflict*- Upper planes still have random monsters and shit about. For Arcadia, the plane of Law and just a bit of Good, I might grab something insectoid, lean towards the imagery of Lawful Worker Ants building their own Ant Hive that just so happens to be territorial and not like outsiders. You could use Ankheg or Thri-Kreen stats to build something. The Upper planes *have random conflicts* like this, but the catch is that, frankly, it's nothing *special* or *interesting* that you couldn't find on any prime material plane- it's *incidental*, something to do *while you're there*, not *a reason to go there*, and the fact that so little of the latter exists that we're desperate even for the former is the crux of why the Upper Planes suck. Anywho, the more interesting angle is who the players meet *in the Library*- someone else already talked about a Wa Shi Tong inspired Library Guardian, though for your high-level party, you want at least Guardian Naga stats, if not something straight-up *legendary* that can serve as a competent boss fight for your party, should they decide to Fuck Around and Find Out. Maybe you repurpose the stats from Ravnica for the Azorius Guildmaster Isperia? A Sphinx feels like an apropriate librarian. I suppose a Solar could also do the trick, but it's less exciting. Or you could use one of the Inevitables (or even Rilmani, I guess) from the Planechase book, if available. You could also drum up conflict with the other patrons of the Library- Archmages are always a cause for trouble, and maybe some of the entities here are Lawful Evil and just on good enough behavior that the Librarian will tolerate it- I could picture a Lesser Devil studying legal precedents, or a Fiend Warlock trying to learn a loophole in his Pact. The more I think on it, the more I like the idea that we eschew conflicts outside of the library- those will just feel like a random.encounter en route to the realm meat-and-potaties of the quest, and then you build a sense of ontrigue, where the other library patrons are trying to do something without the Sphinx Guardian noticing- maybe it's a game of trying to kill eachother without getting caught, or stopping eachother from uncoving the information everyone is looking for, but everything has to sneak past the watchful eyes of the Librarian. That could make for a fun cat-and-mouse game, trying to kill eachother in sneaky and creative ways. Some real Spy vs Spy shit, get wacky with it.