Consider the following oddities.
1) We find traces of Pseudo-Isidore’s activity here and there throughout the 840s. The clearest example are the fingerprints he left on the decrees of the Council of Meaux/Paris from 845. Several capitula rehearse Pseudo-Isidorian ideas about chorbishops, the consecration of chrism, and the integrity of church property. They do so vaguely and distantly, filtered through somebody else’s prose, and qualified by other points of view. After the later 840s, traces like these, of a living breathing Pseudo-Isidore with committee assignments, evaporate and never return.
2) The picture from inside is exactly the same. The forgeries respond to events from the reign of Louis the Pious, particularly the Council of Thionville from 835. Much of their procedural law strives to exclude episcopal depositions like those enacted there. As one progresses from the earliest forgeries (the Hispana, Benedictus Levita Book I/Book II) to the later forgeries (especially the False Decretals), the emphasis shifts. Not only is Thionville to be prevented in the future; it is to be invalidated in the moment. The earlier forgeries were as abstract as possible, even if the ultimate point of their provisions was clear. With the False Decretals, Ebo comes suddenly into focus, as the forgeries angle to reinforce the legality of his installation at Hildesheim. Hincmar, too, is suddenly present. The forgeries deplore his consecration as archbishop of Reims while Ebo still lives in exile. An anti-metropolitan aspect that is clearly about Hincmar becomes more defined.
3) The latest event that the forgeries seem to know about is the death of Archbishop Otgar of Mainz in 847. The preface to the False Capitularies famously characterises Otgar as the former archbishop of that see, in such a way as to suggest that he’s been dead for a little while at least. After the False Decretals it appears that Pseudo-Isidore never forged anything else again, and no items associated with Pseudo-Isidore reveal the slightest awareness of anything that happened after about 850. The Council of Soissons from 853 leaves no impression on any Pseudo-Isidorian invention at all, and if anything were to inspire Pseudo-Isidore to take up his pen again, you’d think it would be Soissons 853.
4) There are a lot of signs that Pseudo-Isidore never put the finishing touches on his masterpiece, the False Decretals. Quite obviously, no fair copy was ever produced, and the full collection was assembled from its constituent components on at least two distinct occasions, by people who were less than fully acquainted with its intricacies. The preface of Isidorus Mercator, for example, describes a three-part collection that differs in slight but important ways from the long versions that have survived. Strange mistakes in arrangement and variations in contents plague both A1 and A/B, with independent errors occurring in both. A1, for example, messes up some Pseudo-Isidore’s corrections to the sequence of decretals in the Hispana Gallica, and includes one or two forgeries that for intriguing reasons seem not to belong, almost as if they were early, discarded efforts. A/B rightly omits these, but has errors of its own: It messes up the Symmachus dossier and puts the false Fifth and Sixth councils at the end rather than where they belong. Its architects are also confused, for example, about how the prefatory material from the Hispana is to be integrated with the prefatory material the forger has invented, and they end up including the Hispana preface right after the Isidorus Mercator preface, even though the latter is an expansion of the former. Errors and anomalies of arrangement like this are what the different recensions are made of.
5) The forgeries were not immediately circulated. In 852, Hincmar alludes to two decretal forgeries in regulations for his clergy, but it’s not clear how well he knows these documents. Perhaps also around this year, Archbishop Thietgaud of Trier begins to think that as metropolitan of Belgica Prima (Trier), he enjoys Pseudo-Isidorian primacy over Belgica Secunda (Reims). He has heard somewhere about Pseudo-Isidore’s totally novel idea of primacy. After those two hints, we have nothing until 857, and then again the first person to cite the documents is Hincmar at Quierzy, now verbatim. Only after 857 do the forgeries really seem to be out and about. Horst Fuhrmann discussed this delay with a citation from Johannes Haller: “Man fälscht nicht auf Vorrat”: You don’t forge things just to stockpile them.
To summarise: Pseudo-Isidore, whoever he was, clearly took part in the life of the Reims province in the 840s but he disappears after 850. His forgeries also belong to this decade and are aware of nothing that happens afterwards. The False Decretals in particular appear to have been assembled in their final form by people with an imperfect understanding of how they fit together. When they were finally released, after considerable delay, they came first to the eyes of Pseudo-Isidore’s greatest enemy (Hincmar), and were too late to help Ebo (who died in 851) or the clerics he had doubtfully ordained (who were deposed in 853).
Everyone has to stop writing their book sometime, but Pseudo-Isidore didn’t just come to the end. He disappeared entirely, for he was unable even to give final form to his latest and most complex creation, he never shepherded his work into the wider world, and as far as we know he never forged anything ever again.
Around 850, it seems, Pseudo-Isidore was abruptly removed from his own undertaking. Death would be the simplest assumption, but he might also have been demoted or deposed or driven out of whatever position he held. If something induced him to abandon his forgeries voluntarily, such as a promotion to higher office, we have to ask why we never find his ideas recurring beneath of the veneer of later conciliar pronouncements and the like, as we do in the years before 850.