He was poisoned in Zeugminon by Thomas 119, who was paid by Istvan's Hungarian enemies; when he was being bled, Thomas placed on the wound a bandage soaked in poison, which spread through his body and killed him
Niketas Choniates, Historia 128.4-10
His body was abused and long left without funeral rites and unburied outside the gate of Zeugminon, till natural instincts prevailed and he was buried in the church of Saint Stephanos the Protomartyr
Kinnamos 239.21-240.3
Was poisoned in Zeugminon by some of his own troops who had been bribed