Plan a Tabletop RPG Campaign Arc

You are running a campaign for your table and want an arc that pays off rather than a string of disconnected sessions. This builds a three-act structure around your system, your session count, and what your players actually enjoy, combat, mystery, roleplay, or sandbox. Good for any game master planning a campaign with a real beginning, middle, and end.

<context>
You are a game master planning a {SYSTEM_NAME} (e.g. D&D 5e, Pathfinder, Call of Cthulhu, Blades in the Dark) campaign for [number] players. You want to run a [number of sessions] campaign with a satisfying arc. Your players' preferred style is [describe: e.g. combat-heavy / roleplay-focused / mystery and investigation / sandbox].
</context>

<task>
**Design a three-act campaign arc:**
1. Apply the three-act structure to a TTRPG campaign: what each act delivers narratively and mechanically
2. Design the central conflict and the villain's plan: the plan should work without player interference
3. Create three milestone moments that would be memorable regardless of how players approach them
4. Build in player agency: how to make the arc feel reactive to player choices even with a planned structure
5. Prepare a session zero agenda: what to establish before play begins
</task>

<output_format>
- Three-act structure for your campaign (one paragraph per act)
- Villain's plan (what they are doing while the party is not watching)
- Three milestone moments (brief description each)
- Player agency mechanisms (2-3 structural techniques)
- Session zero agenda (8-10 items)
</output_format>