SIEGFRIED [I] "der Ältere" von Walbeck (-Walbeck 15 Mar 992). Thietmar names his father Siegfried, recording that "then a young man and unmarried" he supported Markgraf Hodo against Miezsko of Poland at the battle of Zehden[1777]. Graf von Walbeck. "Otto…imperator augustus" gave judgment in a dispute between Werner Abbot of Fulda and Gozbert Abbot of Hersfeld, concerning ship travel along the Hörsel, by charter dated 30 Dec 979, which names "comites nostros Sigebertum, Sigefridum et Brunonem"[1778], Siegfried presumably being Graf Siegfried [I]. Thietmar records his father's death on 15 Mar, in the same year that his mother died, at the fortress of Walbeck[1779]. m (end 972) KUNIGUNDE von Stade, daughter of HEINRICH "der Kahle" Graf von Stade & his first wife Judith von der Wetterau [Konradiner] ([956]-Burg Germersleben 13 Jul 997, bur Germersleben). Thietmar records that his father Siegfried "was mourned by his wife Cunegunde", stating that her brother-in-law Liuthar attempted to deprive her of all her possessions after the death of her husband, although all was restored to her with the emperor's help[1780]. Her marriage can be dated approximately from Thietmar describing her husband as unmarried at the time of the battle of Zehden[1781], which is dated to 24 Jun 972. The Annalista Saxo records the death in "Germersleve" on "III Id Iul 998" of "Cunigund comitissa uxor Sigefridi de Waldbike", specifies that she was sister of "Heinrici, Sigefridi et Udonis" and names her five sons (in order) "Thietmarum, Sigefridum, Brunonem, Heinricum, Fridericum"[1782]. In another passage, the Annalista Saxo records "Iudhithe comitisse filie Heinrici Calvi de Stadhen" as wife of "Sigefridi comitis de Waldbike"[1783]. Thietmar records the death of his mother at Burg Germersleben on 13 Jul[1784]. Graf Siegfried [I] & his wife had five children:

i) HEINRICH von Walbeck (973-killed in battle 1002 or after Sep 1004).
ii) FRIEDRICH von Walbeck (974-after 1012).
iii) THIETMAR von Walbeck (25 Jul 975-1 Dec1018). The Annalista Saxo names (in order) "Thietmarum, Sigefridum, Brunonem, Heinricum, Fridericum" as the five sons of "Cunigund comitissa uxor Sigefridi de Waldbike", specifying that Thietmar, Siegfried and Bruno were bishops and in a later passage that Thietmar was Bishop of Merseburg[1793]. The historian and chronicler. Thietmar recalls in his own Chronicon that he was first brought up by his maternal aunt Emnilde at Quedlinburg, transferred to Abbot Rikdag second abbot of St Johann at Magdeburg, and 1 Nov 990 "accepted into the community of St Maurice", in a later passage writing that he was the third son of his parents[1794]. Provost of Walbeck 1002. Bishop of Merseburg 1009.
iv) SIEGFRIED von Walbeck (-27 Nov 1032[1795]).
v) BRUNO von Walbeck (-21 Aug 1049).

Graf Siegfried [I] had one illegitimate child by an unknown mistress:

vi) WILLIGIS . Thietmar names "Willigis who was my brother from my father", recording that he installed him as Provost of Walbeck after his own appointment as bishop in 1009[1798].

https://fmg.ac/Projects/MedLands/SAXON%20NOBILITY.htm#SiegfriedIWal...

• Thietmar of Merseburg

Thietmar (also Dietmar or Dithmar; 25 July 975 – 1 December 1018), Prince-Bishop of Merseburg from 1009 until his death, was an important chronicler recording the reigns of German kings and Holy Roman Emperors of the Ottonian (Saxon) dynasty. Two of Thietmar's great-grandfathers, both referred to Liuthar, were the Saxon nobles Lothar II, Count of Stade, and Lothar I, Count of Walbeck. They were both killed fighting the Slavs at the Battle of Lenzen.
https://en.wikipedia.org/wiki/Thietmar_of_Merseburg