The Directorium inquisitorum was written by Spanish theologian Nicholas Eymeric (c. 1320-1399), who was appointed grand inquisitor of Aragon in 1357. Intended to be a guide for inquisitors, the Directorium inquisitorum elaborates on hundreds of heresies and prosecution procedures, categories of offenses like witchcraft, as well as the belief system of the Inquisition. It also influenced later texts such as the Malleus maleficarum (Hammer of witches, 1486), and remained an important volume well into the 17th century.
The text was first printed as early as 1376. This edition, printed in 1578, is the second Italian printing, and third edition overall. It includes commentary by Francesco Pegna (Peña, c. 1540-1612), an Aragonese canonist with connections to the Roman Curia.