Once again, unfaithfulness is more of a motive than a means, but it is a good story anyway.
Let me introduce you to yet another Hungarian king: László IV (1272-1290), nicknamed " the Kun" (Cuman). He came from a strange family: Grandson to the king that weathered the Mongolian Invasion, nephew to St. Margaret of Hungary, son to the next king and a Cuman princess, one of 7 children. Half Christian, half nomadic, slightly confused, completely wild.
It was a turbulent time in the history of the Hungarian kingdom. László was not a strong ruler, and he was influenced by several noble families that all struggled for power. The king found refuge with his mother's people the Cumans who were allowed to settle within the kingdom after the Mongolian Invasion (and as a preemptive ally against a possible next invasion - which is why they were allowed to marry into the royal family).
On top of all of this, the Mongolians attacked the second time; it was not as devastating as the first invasion, but still pretty bad (also some people claimed that László himself called them in this time - he did have a thing for nomadic cultures).
The irony of the troubled king's life is reflected in his death: He was assassinated in his sleep by three Cuman men. He was the second to last ruler in Hungary's first, 300-year-long royal dynasty.