From the introduction: Ouroboros is a rules engine for tabletop roleplaying in fantastic worlds. Like all tabletop systems, it makes certain assumptions about how a group will use it... ... The Ouroboros Engine is a general-use system for fantasy gaming, with just enough density for both tactics and flexible stakes, but is otherwise light. ... When a situation comes up where a roll is appropriate, the Guide will note what you're trying to do, and will describe what might go wrong, and how you might do well. You will then roll twenty-sided dice equal to an applicable skill, and each die that comes up equal to or less than the appropriate attribute is “one success”.