What is "sfengus"

According to John Skylitzes, Sfengus or Sphengos was a brother of Knyaz Vladimir I of Kiev. Sfengus was a leader in the joint Byzantine- Kievan campaign to depose Georgius Tzul, the last recorded khagan of the Khazars. Though identified as a brother of Vladimir I of Kiev, some historians such as Simon Franklin and Jonathan Shepard hypothesize that he is identical with Mstislav of Tmutarakan, Vladimir's son.