The game master’s guide. It includes:

- GM tools;
- adventuring rewards;
- how to run monsters;
- monster builder;
- bestiary (69 monster statblocks);
- legendary monsters (21 monster statblocks);
- starter adventure;
- a sandbox-style setting;
- detailed locations (with plots and hooks);
- seven one-shot adventures;
- rules for converting 5e content.

This is book 2 of 3 that you need to play Nimble.