Ahriman, greatest sorcerer of the Thousand Sons and architect of the Rubric that laid his Legion low, continues to walk the path towards salvation, or damnation. Searching for a cure for his Legion, he is forced to consider – was the great ritual somehow flawed from the very beginning? The answer may lie within the mysterious artefact known as the Athenaeum of Kalimakus, a grimoire of forgotten knowledge that is reputed to contain the exact words of the lost Book of Magnus... or, perhaps, even a transcription of the primarch's deepest and most secret thoughts.