From the Publisher' blurb:
Heart of St. Bathus is a sandbox-style campaign for OSE roleplaying designed to take a party of characters from Level 3 to Level 12. Answering an appeal for aid from the monks of Goodheart Abbey, the adventurers set off in search of a famed paladin’s lost soul, venturing first to a cursed fortress and then into the labyrinthine depths beneath. But it’s far from a simple mission: The soul — and the heart-shaped jewel to which it is bound — have been stolen by a cunning devil who has taken it to her citadel far below within the twisting passages of the Cyclopean Deeps. And the Heart’s true master, an imprisoned demon lord, lurks in the darkest corners of the realm, hoping to use the unwitting adventurers to aid in his escape. The soul of a brave paladin hangs in the balance as the adventurers delve into dark and unknown regions!