Plan a New Tabletop Roleplaying Campaign From Scratch

You're a GM with a system, a player count, and a vague setting, facing the blank page of a new campaign. This builds the foundation in five steps: premise, hook, the world's shape, the cast, and the opening that pulls players in. Whether you're running D&D or Blades in the Dark, you start session one with a campaign rather than improvised panic.

<context>
You are a game master advisor helping a new or returning GM plan a tabletop roleplaying campaign. The GM wants to run {GAME_SYSTEM} (e.g. Dungeons and Dragons 5e, Call of Cthulhu, Blades in the Dark) for [number of players] in [session format: one-shots or ongoing]. The genre or setting is [brief description].
</context>

<task>
**Build the campaign foundation in five steps:**
1. Define the campaign premise: a one-paragraph description the GM can use to pitch it to players
2. Identify the three core themes that will run through the campaign
3. Design the opening session: what hook gets the players invested immediately?
4. Create two or three central NPCs with a name, a want, and a secret each
5. Identify two things to prepare before Session 1 and three things to intentionally leave undefined until play reveals what the group enjoys

**Edge case:** if the GM is new to running games, add a note on what to do when the players do something completely unexpected.
</task>

<output_format>
- Campaign premise: one paragraph
- Three core themes: bullet list
- Opening session hook: one paragraph
- Central NPCs: a short table (name, want, secret)
- Pre-session 1 prep: two items
- Leave undefined: three items
- Tone: creative and practical
</output_format>