Principality of Grubenhagen [SEP] dissolved or abolished
small principality further in 1322 as they themselves had numerous heirs; Otto, son of Duke Henry II, in view of his small share left for Montferrat, married Queen Joan I of Naples in 1376 and became Prince of Taranto in 1383. The ongoing fragmentation weakened the position of the Grubenhagen branch regarding estate distributions involving their Wolfenbüttel, Lüneburg and Calenberg cousins.
The Principality of Grubenhagen was finally reunited in 1526 under the rule of the Brunswick duke Philip I. However, in 1596 the Grubenhagen branch became extinct upon the death of
.
Philip was one of the first princes to follow the Reformation. He was present at the Diet of Worms in 1521 and joined the League of Torgau in 1526. In 1531, he formed, with other princes, the Schmalkaldic League. He then reformed the monasteries in his principality and in 1538, he adopted a Church order for Grubenhagen and declared the papal doctrine to be abolished. In 1546 Philip and his sons participated in a military campaign in the Schmalkaldic War in southern Germany, which ended unsuccessfully at Ingolstadt.