King Béla started to groom his first born son, István (who was an only son for a long time), into a king very early, from not long after the years of the Mongol invasion (1241-42). Then he shared the weight of ruling with him as a iunior rex in about the last decade of his life. However a split happened between the two kings, the old and the young, and a civil war broke out. By then, our hero Béla, son of Rastislav, was already a Ban in Macsó. He sided with his namesake, his grandfather, and led a body of his troops into battle where the senior king's army suffered a defeat, the duke of our story managed to escape but another leader, a very powerful lord, a certain Henrik of Kőszeg, from the gens Héder, Ban of Slavonia and literally the first oligarch in the country, fell into the hands of István.
After the death of king Béla, Henrik fled the country to Bohemia where Otakar II's wife, Kinigunda, was the daughter of the same Anna who gave birth to duke Béla. Not just Henrik fled, but other nobles and most notably the queen mother and Anna too. However duke Béla asked for and got pardon from the sole new king István V. And Indeed, soon on his side he went against the Czech king on a campaign when he attacked the country. The king took away some titles and many holdings of Henrik.
But soon István died, and his son, László IV was only ten years old who couldn't rule on his own right yet. His mother, Elizabeth, became the governor and guardian - in theory. There was only one adult male relative, duke Béla so he was considered as a candidate for those roles - in theory. In practice however two cliques of aristocratic families started to compete for the control over the child king, one was formed around the gens Csák, the other around the family of Kőszeg (the gens Héder) with the now returned Henrik at the helm.
The royal court frequently spent time on the Margaret Island in the middle of the Danube near Buda (there weren't any Budapest back then), where duke Béla's younger sister lived in the nunnery. Sometimes nobles visited the court there just as happened in the November of 1272. But then happened something else. Henrik accused Béla with treason, maybe citing that his relatives rule Bohemia and he wants the Hungarian Kingdom to himself, quarrel ensued then Henrik with his retainers attacked the duke and cut him into minced meat. Henrik suffered no blowback which illustrates well the weakness of the royal power.