The Directorium Inquisitorum is Nicholas Eymerich's most prominent and enduring work, written in Latin and consisting of approximately 800 pages, which he had composed as early as 1376.

Eymerich had written an earlier treatise on sorcery, perhaps as early as 1359, which he extensively reworked into the Directorium Inqusitorum.

As he directed much of his efforts to the apparent errors of members of the clergy, he often found his investigations blocked by the court, curia, or papacy.

Finally in 1376 he drove him from the kingdom and Eymerich fled to the papal court of Pope Gregory XI in Avignon.

[3] The Directorium Inquisitorum includes definitions of various types of heresies, discussion of questions of jurisdiction, and proper trial procedure.

[2] On the subject of magic, it discusses various types and techniques of divination and draws a distinction between practices deemed heretical and non-heretical.