Translated from the back of the book:
More than fourhundred years ago Rohal banished his brother Borbarad into the void of the limbo. Rohal, the greatest wizard of all time and respected monarch, too wasn't seen ever since. But now Borbarad has returned from his prison between the spheres and covers the land in murder and fire; and nothing seems to be able to stop him.
But don't all records state that Rohal promised to return in the hour of greatest need?
Rohals Promise is part five of the campaign about the Marks of the Seven and Borbarad's return."
Additional user summary (contains spoilers):
Part 5 of 7 of the campain "The Marks of the Seven" about the return and downfall of legendary black wizard and demon master Borbarad.
In part five the heroes travel from a magical convent to the promised return of Rohal who sends them on a quest to find the time Borbarad has stolen incarnated in a child. Rohal then meets his brother Borbarad with a surprising outcome.