He was the eldest son of Geoffroy I of Semur (c. 942 - c. 1000), lord of Semur-en-Brionnais, and his first wife, a daughter of Dalmace II, Viscount of Brioude (c. 935/950 - 985).
It is unclear which method was used to kill Dalmas, but Hildebert de Lavardin, a contemporary, reported that the Duke killed his father-in-law "propia manu," which has led some historians to suppose Dalmas lost his life in battle over the territory around Auxerre.
However, given that Dalmas's son Josserand was also killed by "two of the Duke's soldiers" while trying to break up a fight between Robert and his father, it seems the scene of the murder may have been more intimate.
[1][2][3] It has also been theorized - based on figures on the tympanum on the gates of Notre-Dame de Semur-en-Auxois, built sometime after 1250 - that Dalmas was poisoned during a banquet attended by his son-in-law.
[4] The construction of Notre-Dame de Semur-en-Auxois was funded by Robert I, and some wonder whether he did so in order to assuage his guilt and atone for killing his wife's father.