He was born in Girona, in Catalonia, and entered he local Dominican monastery while a teenager, learning theology there before being sent to Toulouse and then Paris to further his education. He then returned home to become the theology teacher at the monastery.
His knowledge was so recognized that in 1357 he was named the Inquisitor General of Aragon. In his vigorous pursuit of heretics, he targeted many fellow clerics for small details that he considered blasphemous, earning himself many enemies in the Church. When he decided to interrogate a well-respected Franciscan, Nicholas of Calabria, King Peter IV of Aragon arranged to have him removed from his position in 1360.
The Dominican Order decided that Eymerich would be a good Vicar General, but there was opposition, notably from King Peter IV, who supported a different candidate, Bernardo Ermengaudi. The dispute required the pope to make a decision, but Urban V chose a compromise candidate, Jacopo Dominici.
Eymerich remained an Inquisitor General, further annoying the king by attacking the Ramon Llull's teachings. (One of his objections to Llull was that Llull believed in the Immaculate Conception of Mary while Eymerich did not.) The king forbade him from preaching in Barcelona, but Eymerich became political, not only ignoring the king's command but also supporting a revolt against him in 1376. When the monastery where Eymerich was hiding was surrounded by 200 horsemen seeking him, Eymerich fled to Avignon where Pope Gregory IX was residing.
While in Avignon, he justified his approach to the position of Inquisitor by writing the Directorium Inquisitorum, the "Directory of Inquisitions" with his definitions of heresies, trial procedures, and proper jurisdiction of the inquisitor. He discusses how to find witches and the actions that are considered parts of witchcraft and therefore heretical: casting salt into a fire, burning bodies of animals and birds, baptizing images, mixing names of angels and demons, etc.
Armed with this clear explanation of why he was right in his actions, he decided to return to Aragon in 1381, only to discover that Ermengaudi had become Inquisitor General in his absence. Ignoring this turn of events, he decided to continue acting as if he were Inquisitor General. This did not work well for him. I'll explain further next time.