Rulers of medieval Walbeck in present-day Saxony-Anhalt
The Counts of Walbeck ruled a medieval territory with its capital Walbeck northeast of Helmstedt in the present town Oebisfelde-Weferlingen in Saxony-Anhalt. The foundation of the Archbishopric of Magdeburg established the region as firmly in the oversight of Otto the Great, Holy Roman Emperor. The first Count of Walbeck, Lothar I, was great-grandfather of Thietmar, Prince-Bishop of Merseburg, chronicler of the Ottonian dynasty of Germany and the Holy Roman Empire. Two of Thietmar’s great-grandfathers, both named Lothar, were killed in the Battle of Lenzen, pitting the forces of Henry the Fowler against the Slavs. The early Margraves of the Nordmark were descended from the House of Walbeck.
There were close relationships, and rivalries, between the Counts of Walbeck and the Counts of Stade. The family tree of the Counts of Walbeck is provided in Warner’s Ottonian Germany, a translation of the Chronicon of Thietmar of Merseburg. An excellent source of information is the German Wikipedia article 
The Counts of Walbeck were:
After Conrad, the Counts of Walbeck transitioned to the Margraves of Nordmark (Northern March). Helperich, grandson of Conrad, is also listed as a Count of Walbeck, but the title was likely ceremonial.
Sources.
Warner, David A., "Ottonian Germany: The Chronicon of Thietmar of Merseburg", Manchester University Press, Manchester, 2001
Heinrich, Leo : "Lectures on the History of the German people and Reich", E. Anton, 1867. 
Big, Walthe,: "The Counts of Walbeck", Resin Magazine, 1952
Leyser, Karl, "Medieval Germany and Its Neighbours 900-1250", The Hambledon Press, London, 1982
Bury, J. B. (editor), "The Cambridge Medieval History: Volume III, Germany and the Western Empire", Cambridge University Press, 1922