The Book of the Horsemen is an artifact that consolidates or grants the title of Horseman of War.
In the finale, the group debated whether to give the Book to Auro so that he could destroy the other three Horsemen. For: with full power he eliminates the competition. Against: it makes him more dangerous and risks wider genocide. Luca settles the matter by swearing to execute Auro regardless.
It appears in the finale of Zaira and at Mount Analogue.