Background
Eric was the only son of the Duke Albert I of Brunswick-Grubenhagen, who died in 1383, and his wife Agnes I, a daughter of Duke Magnus II of Brunswick-Lüneburg.
Eric was the only son of the Duke Albert I of Brunswick-Grubenhagen, who died in 1383, and his wife Agnes I, a daughter of Duke Magnus II of Brunswick-Lüneburg.
From 1402 they joined ruled the principality and Eric moved his residence to Salzderhelden. All these disputes were soon settled again. In 1406 Eric was captured in a feud with the Lords of Hardenberg in Lindau.
He was only released after he gave them a written Urfehde and the city of Osterode am Harz as collateral.
In 1415 Eric declared war on the Counts of Hohenstein, because both sides claimed the county of Bad Lauterberg im Harz and defeated them near the village of Osterhagen. Count Guenther of Hohnstein died during the battle.
Counts Henry and Ernest were captured and had to pay 8000 florins and renounce their claims to redeem themselves. A dispute with the Lords of Bortfeld was settled soon after Eric entered into an alliance against them with the City of Brunswick.
A dispute with Landgrave Frederick "the Peaceful" of Thuringia about the city of Einbeck was also settled quickly.
In 1424, the Abbess of Quedlinburg Abbey, Countess Adelheid of Isenburg), enfeoffed the Dukes of Grubenhagen with Duderstadt, Gieboldehausen and the Golden Mark. Eric was married to Elisabeth, a daughter of Duke Otto I "the Angry" of Brunswick-Göttingen.
