Eireann and the Isle of the Mighty is a setting (or "World Unit") for the Pathwanderer Rôle-Playing System. It presents the British Isles during the time known as the Dark Ages, from the end of the Roman invasion in Britain to the beginnings of England as a united country. The setting presents an alternate history where legends are real. It is the age of Merlin and Arthur, of Hengist's early Saxon settlements of Vortigern's Tower and of the wars of the vigorous pagan king Penda. It is an age of magic. The islands are home to a vast number of Druids, monsters, warriors, and gods. Campaigns can be set in Eire (Eireann) and/or Britain (Ynys Prydein, the Isle of the Mighty) in any period from 410 - 796 AD. In addition to the setting details, the document provides "tweaks" that should be made to the core rules to allow campaigns to function smoothly.