Catalyzes the hydrolysis of 3-deoxy-D-manno-octulosonate 8-phosphate (KDO 8-P) to 3-deoxy-D-manno-octulosonate (KDO) and inorganic phosphate. 3-deoxy-alpha-D-manno-2-octulosonate-8-phosphate + H2O = 3-deoxy-alpha-D-manno-oct-2-ulosonate + phosphate Homotetramer. Cobalt was used in the crystallography experiment but magnesium is likely to be the physiological metal. Belongs to the KdsC family.