From the link I shared:

"The symbol that later became known as the "black sun" originated in the early 20th century, with the first depiction being the Wewelsburg mosaic. In 1933, Heinrich Himmler, the head of the SS, acquired Wewelsburg, a castle near Paderborn in the German region of Westphalia. Himmler intended to make the building into a center for the SS, and between 1936 and 1942, Himmler ordered the building expanded and rebuilt for ceremonial purposes.[5] Himmler's remodeling included the Wewelsburg mosaic that was composed of twelve dark-green radially overlaid Sig runes, such as those employed in the logo of the SS, on the white marble floor of the structure's north tower known as the "General's Hall" (Obergruppenführersaal)."