The Chronicon Varadiense names "dux Stephanus postea rex, secundus…dux Bela" as the two sons of "rex Bela quartus". The Chronicle of Alberic de Trois-Fontaines records the birth "in festo beati Luce 1239" of "regi Hungarie filius masculus…Stephanum". The Altahenses Annales name "Stephanum filium [Belæ regis Ungarie]" when recording his succession to his father.
His father appointed him Prince of Transylvania in 1257. He led the Hungarian troops which invaded Bulgaria in 1261, restoring his brother-in-law Rostislav, who was one of the claimants to the Bulgarian throne after Tsar Koloman II was deposed in 1258, at Vidin. Duke of Slavonia, Dalmatia and Croatia, his father transferred these territories to his younger brother Béla in 1262, whereupon István revolted, although peace was concluded 5 Dec 1262 under which the country was divided and István retained the territory north of the Danube along the border with Bulgaria with the titles "rex iunior" and "dominus Cumanorum". Civil war broke out again in 1264, peace once more being confirmed in István's favour in 1266.
He succeeded his father in 1270 as ISTVÁN V King of Hungary.
The Altahenses Annales record the death "1272 circa Kal Aug" of "Stephanus rex Ungarie". The Chronicon Dubnicense records the death "in magna insula" of "Stephanus" and his burial "in ecclesia beate Virginis in insula Budensi in loco Beginarum". The Gesta Hungarorum records that King István was buried "in insula…Beatæ Virginis". 1
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