Abbot of the monastery of St. John in Lübeck from 1177 until his death, and author of the Chronica Slavorum, intended as a continuation of the earlier work of Helmold of Bosau, and covering the years 1172-1209.
The Chronica is an important source not only for German and Danish operations against the Slavs and the advance of Christianity in the region to the east of the river Elbe, but also for the crusade in the Holy Land. It gives lengthy accounts of the pilgrimage to Jerusalem in 1172 of Henry the Lion, duke of Saxony (to which much of Book 1 is devoted), of Sal- adin’s capture of Jerusalem in 1187 (stressing the divisive effects of the quarrel between Guy of Lusignan and Raymond III of Tripoli), and of the Third Crusade (1189-1192), the Crusade of Henry VI of Germany (1197-1198), and the Fourth Crusade (1202-1204). This last section includes a long letter from Baldwin IX, count of Flanders (one of several versions sent to the West), describing the siege and capture of Constantinople (mod. Istanbul, Turkey). There is also a description (out of sequence toward the end of the work) of an embassy sent by Frederick I Barbarossa, Holy Roman Emperor, to Saladin in 1175.
Arnold was a supporter of the Welf family, praised its leader Henry the Lion as another Solomon, and in his account of the duke’s pilgrimage portrayed him as an influential figure who was treated with respect even by the Byzantine emperor Manuel Komnenos. Arnold also translated the Gregorius of Hartmann von Aue from Middle High German into Latin for Henry’s son William of Brunswick.