Johann Weyer was a physician, not a magician, and De Praestigiis Daemonum, the larger work Pseudomonarchia Daemonum appeared alongside, was written to argue against the witch trials sweeping sixteenth century Europe. Weyer's position was that the women and men accused of demonic pacts were, in most cases, suffering from delusion or melancholy rather than genuinely commanding infernal power, and that the courts executing them on confession evidence were killing the mentally ill rather than punishing real sorcery. The demon catalog sits inside that argument as evidence of a different kind: proof, Weyer intended, that the elaborate hierarchy conjurers claimed to command was itself a documented fiction, copied from older manuscripts, rather than a genuine spiritual reality.
The catalog itself lists sixty nine demons in descending rank, from Bael, said to rule in the east, down through dukes, marquises, presidents and knights, each entry giving a name, a description of the shape the demon supposedly takes when it appears, its number of attending legions, and the specific powers, from teaching languages to revealing hidden treasure to raising storms, a conjurer could supposedly command from it. Weyer drew this material from earlier manuscript grimoires rather than inventing it, presenting it less as his own magical knowledge than as the kind of nonsense he believed a rational reader should recognize as nonsense.
That framing did not survive the text's later reception. Readers interested in the catalog for its own sake extracted it from Weyer's skeptical argument entirely, and it went on to a life of its own inside the ceremonial magic tradition he had written against.