Severinus of Sankt Wendel is known as the herbalist at the abbey.  An herbalist, of course, is a person who is skilled in the harvesting and collection of medicinal plants.  Severinus is no exception in that he has extensive knowledge about (and access to) these medicinal herbs in question, as well as some more controversial types that are very poisonous.  Part of his "job" is to supply Malachi with the vision-creating herbs to be given to anyone about to enter the mysterious library.  As a result of Severinus' herbalist work, he unknowingly gives Jorge the poison that fouls the book on comedy by Aristotle.  Severinus is eventually killed indirectly because of his skills as an herbalist, for Malachi (the murderer) has a serious intent to steal that very book by Aristotle.