From the introduction: Siren, the Simple RPG Engine, is a free and open source RPG system that aims at being simple and easy to learn without also compromising on the expressiveness of the system. An RPG engine, much like a video game engine, is a framework on which you can build your own system. If any of the rules in this seem like they don't quite do what you need, you're free to change them or even add new ones if that better suits your campaign. Publisher's blurb: This rulebook covers: Game Creation Character Creation Character Traits Character Skills Character Exploits Combat Character Progression Magic Systems Hacking the Rules It also features an extensive list of examples for things that might be unclear to a new player.