This paper discusses a passage from the poem De triumphis ecclesiae by John of Garland, a medieval intellectual and professor in Paris, compiled in ca. 1252, and referring to the Mongol invasion and the death of the enygmatic “leader of Thrace”. In the text, it is concluded that the enigmatic person, mentioned by the author, is the Bulgarian ruler John Asen II (1218-1241). Along with the writings of Cistercite monk Alberic of Trois-Fontaines, chronicler Philippe Mouskes and Byzantine historian George Akropolites, the poem of John of Garland is the fourth source in which the death of the Bulgarian ruler is recorded. Besides, it is the only text in which the death of John Asen II is associated with the Mongol invasion.