Kayzk the Befouled
Kayzk the Befouled is a champion of Nurgle and one of Tamurkhan's lieutenants. Kayzk himself is little more than a mass of rotting flesh held together by the power of Nurgle, sitting atop a rotbeast, a half-daemon creature of cancerous flesh.
Before joining Tamurkhan's horde, Kayzk lead a group of Kurgan horsemen called the Rot Knights, who also rode rotbeasts, and gave his body and soul to the Plaguefather. Upon joining Tamurkhan, he quickly became his top lieutenant and master of the mounted troops. He fell during the Battle of the Black Pit, having been killed by the sorcerous flames of Drazhoath the Ashen as he tried an ill-judged attack to secure slaves for the horde.
He's back in Age of Sigmar, having probably been resurrected by Nurgle.