This is a study of an apocalyptic Latin letter (incipit “Ad flagellum humani generis”), surviving in manuscripts from the mid-thirteenth to fourteenth centuries, that describes an apparent aggressive invasion of an ascetic army in the distant East, led by a figure claiming to be Christ and bearing a new volume of scripture. This article offers the first comprehensive study of the letter's manuscript tradition and presents a new critical edition of the text. It argues that this letter was composed in the crusader Kingdom of Jerusalem sometime in the years 1235–36 as a response to intelligence brought by eastern Christian envoys (quite possibly from Georgia or Greater Armenia) concerning the second wave of Mongol invasions in Transcaucasia. These envoys had spent some time in the presence of a Mongol army, possibly that of the general Chormaghan, receiving an edict that probably demanded their submission and stated the Mongols’ divine right to universal domination. This edict, accompanied by other information, was ultimately translated into Latin for the benefit of the authorities of the Kingdom of Jerusalem. These authorities interpreted both the edict and the oral and/or written intelligence that the eastern Christian envoys delivered within the intellectual framework of Latin Christianity. This particular interpretation was then written into a letter that was sent to Western Europe, where it circulated probably quite widely for around a century. Crusade theorists’ need for intelligence about the Middle and Far East, together with the vogue of apocalyptic prophecy in the later Middle Ages, encouraged the continued copying of the text.