Thankmar, half-brother of Otto I (during Siege of Eresburg) (born c. 908)

Thankmar was the eldest son of Henry I of Germany by his first wife, Hatheburg of Merseburg. His mother had been previously married and widowed, after which she entered a convent. Because she left the convent to marry Henry, her second marriage was considered invalid and the couple split. Thankmar's legitimacy was, therefore, in question.

Otto I, known as Otto the Great or Otto of Saxony, was East Frankish (German) king from 936 and Holy Roman Emperor from 962 until his death in 973. He was the eldest son of Henry the Fowler and Matilda of Ringelheim.

The Eresburg is the largest, well-known (Old) Saxon refuge castle (Volksburg) and was located in the area of the present German village of Obermarsberg in the borough of Marsberg in the county of Hochsauerlandkreis. It was a hill castle built on the plateau of a low table hill, known as the Eresberg, at a height of 130–150 metres above the Diemel, a tributary of the Weser, in the extreme south of the Saxon Gau of Engern on the border with the Duchy of Franconia.