GDL-III, a description language for general game playing with imperfect information and introspection, supports the specification of epistemic games. These are characterised by rules that depend on the knowledge of players. GDL-III provides a simpler language for representing actions and knowledge than existing formalisms: domain descriptions require neither explicit axioms about the epistemic effects of actions, nor explicit specifications of accessibility relations. We develop a formal semantics for GDL-III and demonstrate that this language, despite its syntactic simplicity, is expressive enough to model the famous Muddy Children domain. We also show that it significantly enhances the expressiveness of its predecessor GDL-II by formally proving that termination of games becomes undecidable, and we present experimental results with a reasoner for GDL-III applied to general epistemic puzzles.