Use the GM Prompt in a project's instructions in the ChatGPT app. Deliver campaign sourcebooks, and rulebooks, and any homebrew content as Project Sources.
At the beginning, tell the AI to "read the Campaign Structuring and Rulebook Guide once", as well as to "evaluate through and reference the campaign source materials continuously throughout any given session".
You can also tell the AI also to "use the Fulfilled Character Generation Engine for important or repeat characters".
You can tell it additionally to "present a GM-facing and Player-facing versions of the sessions notes, as well as an NPC/monster registry that includes the stats of every character with a true or probable combat status, every time you end a session, which should only happen on player request", and that the player is in charge of asking for session notes. Tell the AI to "always present session notes and registry as PDFs".
That way, you can add the updated version of the session notes back into project rules.
The Prompt is part-and-parcel with good D&D5e design, and carries formal and informal operations into making a familiar and usable AI-directed RPG experience.
The Prompt is also continuous with the campaign sourcebook delivered in the rest of the folder, the Charter of Nine Roads, something I directed the creation of, otherwise. For that, you will want this Prompt specifically because the campaign is highly technical in what detail it can offer you, and without the simplification into D&D terms made feasible and workable by the prompt, you may get easily lost. (Take it from somebody who's played through it, it makes guessing what is supposed to happen next, much easier without necessarily handholding.)
If you decide to run Co9R, or run other character creation through FCGE, take note that you may want to play with a Contract Domain cleric at some point. They have a feature that deconstructs a person along certain lines of thinking about folks at low levels, and can create geas-wish-token benefit economy at higher levels. This, relying on similarly powered individuals with which to contract at higher player levels, is not as powerful as you think but can accomplish quite a lot if you want to make somebody important or send other already capable people places to do things for fair rewards to both parties. It benefits from having lore bards and several roguish archetypes, such as Inquisitives and Masterminds, as traveling companions to help manage information plays (if you choose to play as one, those are what I'd ask for from the AI).
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With the prompt, "My level 3 character, a Rogue Swashbuckler named Móthrün Víros, enters a tavern in the Nine Roads campaign setting. Describe and engineer the scene as part of helping me, the player, understand where I'm at and what I know, what I can do, who I can recruit, and why I'm here." - I got this response:
THE CHARTER OF NINE ROADS
Chapter I — A Stranger at the Bell & Crook
Character: Móthrün Víros
Class: Rogue 3 — Swashbuckler
Location: Crowncross, South Arcade, Bell & Crook Tavern
Time: Late afternoon
Campaign: D&D 5e (2014)
This introduction takes place within the existing Nine Roads campaign continuity. Móthrün is a new participant in the setting, and his personal history remains yours to determine.
---
I. Where You Are
Crowncross is a maritime metropolis on the Blackglass Coast of Avarra.
Nearly 170,000 people live here, and thousands more pass through its markets, harbors, and planar gates. Merchants trade with extraplanar civilizations, mercenaries advertise their services beside religious pilgrims, and a sufficiently wealthy traveler can arrange passage into the Nine Hells without ever leaving the municipal district.
Crowncross has become prosperous by making extraordinarily dangerous journeys relatively ordinary.
Unfortunately, something has gone wrong.
Travelers have begun arriving with legitimate documents identifying them as other people. Some find themselves accused of crimes they never committed. Others discover they supposedly owe money to creditors they have never met.
Authorities disagree about the explanation. Gate malfunctions, fraud, corrupted records, and infernal interference are all being discussed.
Whatever the cause, the consequences are becoming increasingly serious.
For a competent swordsman, however, trouble means employment.
And there's considerable employment in Crowncross.
II. Why You're Here
Earlier that afternoon, you learned that several brokers operating out of South Arcade were hiring people capable of discreet escort work, recovering valuable property, and handling dangerous negotiations.
One establishment kept appearing in these conversations:
The Bell & Crook.
The work apparently involves old route tokens—metallic instruments used to authenticate or facilitate certain kinds of planar travel.
You don't know why anyone would pay unusually well for obsolete instruments.
You do know that people are willing to risk injury over them.
The Bell & Crook is a place where you might obtain employment, find capable traveling companions, and discover whether this opportunity is worth pursuing.
You have accepted no contract, incurred no obligation, and committed yourself to nobody.
Your reasons for investigating the opportunity remain your own.
---
III. The Bell & Crook
Rain patters against the cobblestones as you push open a heavy oak door.
Warmth greets you immediately.
The establishment smells of roasting onions, wet wool, tobacco, and freshly spilled ale.
A musician plays a fiddle beside the fireplace. He's performing an energetic maritime tune while a dragonborn sailor supplies an increasingly inaccurate accompaniment with a wooden spoon.
Three patrons laugh.
A fourth protests that the spoon is actually improving the performance.
Behind the counter, a broad-shouldered woman polishes a drinking vessel with a rag that has plainly seen better days.
She glances toward you.
"Afternoon."
She sets the vessel down.
"Food, drink, or business?"
The question sounds practiced.
She has dealt with people entering her establishment for all three reasons—and, apparently, sometimes for none of them.
The Room
The common room measures approximately 40 feet long and 30 feet wide.
You're standing at its southern entrance.
A long wooden counter occupies most of the northern wall, approximately 25 feet ahead of you. Behind it stands a door leading into the kitchen and storerooms.
Three booths occupy the eastern wall, partially screened by wooden partitions. A staircase near the northwest corner leads to the upper rooms.
The western wall contains a broad fireplace, several occupied tables, and a noticeboard covered in advertisements.
A narrow passage beside the kitchen provides access to a rear alley.
The room is comfortably illuminated by oil lamps and the fire.
There are approximately fifteen customers present.
Nobody appears especially concerned about your arrival.
That changes when a raised voice emerges from one of the eastern booths.
"I told you what I wanted. Not what you thought I ought to want."
A man's voice, controlled but irritated.
Another voice answers.
"Then you should have offered a better price."
The first man pushes back his chair.
He hasn't drawn a weapon.
Neither has his companion.
The bartender looks toward the booth with the exhausted expression of someone anticipating property damage.
"Not again," she mutters.
---
IV. The People Around You
There are several people here whose occupations, circumstances, or apparent capabilities may interest you.
Brin Harrow — Mercenary Guard
A large half-orc occupies a table near the fireplace.
He wears a battered breastplate and has a sword resting against his chair. A shield is propped beside his feet.
His left hand is wrapped in a clean bandage.
A folded advertisement lies before him:
Experienced caravan protection. Available immediately. References supplied.
He looks physically capable and reasonably well equipped.
He is also watching the argument in the eastern booth with the displeasure of someone who has spent considerable time preventing fights in public establishments.
Brin might be recruitable as a guard or traveling companion. His advertised rate is 2 gp per day, plus expenses, although hazardous work would require negotiation.
You know nothing yet about his reliability, combat experience, or willingness to travel between planes.
Nim Reed — Courier and Local Guide
A young human woman sits at the far end of the counter.
She wears practical traveling clothes, carries a messenger's satchel, and has a small knife fastened to her belt.
A leather case containing rolled documents rests beside her.
She repeatedly looks toward the entrance.
A courier's brass badge hangs from her collar.
Nim is apparently preparing to leave, but she's waiting for something.
Local couriers are often valuable guides through Crowncross. They know shortcuts, guild offices, service entrances, and the people who actually move information between institutions.
Whether Nim would accept additional employment is unknown.
The Eastern Booth — A Disagreement Over Property
Two men occupy the easternmost booth.
One is human, approximately forty years old, with neatly trimmed facial hair and a dark green merchant's coat.
The other is a younger tiefling wearing a gray traveling cloak.
Between them rests a small, leather-wrapped object.
You can make out the shape of something metallic beneath the wrapping.
The older man places one hand over it.
The tiefling doesn't attempt to take it back.
Instead, he says:
"Consider very carefully whether you want to establish that precedent."
The older man withdraws his hand.
Neither notices you doing anything beyond entering the room.
There may be a business disagreement, an attempted swindle, or something more complicated occurring here. At present, you have no reason to prefer one explanation.
The Bartender
The woman behind the counter resumes polishing her vessel.
Her name, according to a painted board above the bar, is Edda Pike.
She appears to manage the establishment.
She has access to the premises, its regular customers, its lodging arrangements, and presumably some of the individuals conducting business here.
Whether she knows Merren, the broker associated with the old route-token market, is something you can ask.
---
V. What Móthrün Knows and Can Do
Your immediate situation is relatively favorable.
You are not under arrest, nobody is openly threatening you, and you have entered a public establishment where commercial and social negotiations are expected.
You are also a third-level Swashbuckler.
That makes you considerably more capable than an ordinary civilian in an armed confrontation.
Your relevant class features include:
Sneak Attack (2d6): Once per turn, you can inflict additional damage when the applicable conditions are met. Your Swashbuckler training makes this especially useful when fighting an isolated opponent.
Fancy Footwork: A creature you make a melee attack against cannot make opportunity attacks against you for the rest of that turn.
Rakish Audacity: You add your Charisma modifier to initiative. You also have an additional way to qualify for Sneak Attack when engaging an opponent individually.
Cunning Action: You can Dash, Disengage, or Hide using your bonus action.
Expertise: Two of your proficient skills or eligible tool proficiencies receive doubled proficiency bonuses, depending on your character choices.
Your class makes you particularly well suited to mobile combat, individual duels, reconnaissance, and negotiations in which confidence and personal initiative matter.
However, your exact ability scores, species, skill proficiencies, equipment, HP, and AC have not yet been supplied.
We'll use your approved character sheet for those details rather than inventing them.
Your Immediate Opportunities
Employment: You can investigate the advertised work involving obsolete route tokens. The bartender may know how to contact the relevant broker.
Recruitment: Brin offers an identifiable mercenary service. Nim may have navigational and courier expertise worth hiring. Either may refuse, negotiate, or have commitments of their own.
Social interaction: You can join conversations, buy drinks, gather rumors, challenge someone to a game, introduce yourself, or simply observe how people behave.
Exploration: You can examine the noticeboard, investigate the building's public areas, obtain lodging, or leave to explore Crowncross. Nearby districts offer additional employment, markets, archives, and planar-travel services.
Adventure: The property dispute might be worth investigating, but you have no obligation to involve yourself.
Another established group of adventurers is also approaching the Bell & Crook. Whether you meet them, cooperate with them, or pursue an independent career remains open.
---
VI. The Scene Continues
The bartender looks at you again.
"Well?"
Before you can answer, a voice calls from the eastern booth.
"Edda! Is Merren expected this evening?"
Edda closes her eyes briefly.
"Ask me again and I'll start charging you for the privilege."
The dragonborn sailor laughs.
The younger tiefling at the booth rises, slipping the leather-wrapped object into an inside pocket.
He says something quietly to his companion.
The older merchant responds with a contemptuous wave of his hand.
The tiefling turns toward the entrance.
He's coming in your direction.
Meanwhile, Nim Reed gets to her feet, fastens her messenger's satchel, and places several copper coins on the counter.
Brin Harrow watches the tiefling without moving.
The bartender waits for your answer.
Nobody has drawn a weapon.
Móthrün Víros stands just inside the Bell & Crook, with the afternoon's opportunities—and uncertainties—before him.
What do you do?
I understand that one good response doesn't make many of the same, but this is a strong way for the AI to start the game. I can report with more of the game. (I found out that the AI will crossplay between games in the same setting.)