What I'm about to say is not a definitive answer, but serves as more of a long-winded follow-up question. Hopefully it may help.
You may well be conflating history and fiction - at least, it is if you're thinking about the Prince of Pentos the A Song of Ice and Fire series, which almost exactly matches the description you have given.
From Martin, George R.R.: A Dance With Dragons, I: Dreams and Dust, (2011), Tyrion I, p. 33: Magister Illyrio talking to Tyrion Lannister:
"In Pentos we have a prince, my friend. He presides at ball and feast and rides about the city in a palanquin of ivory and gold. Three heralds go before him with the golden scales of trade, the iron sword of war, and the silver scourge of justice. On the first day of each new year he must deflower the maid of the fields and the maid of the seas" ... "Yet should a crop fail or a war be lost, we cut his throat to appease the gods and choose a new prince from amongst the forty families".
Of course, this is a work of fiction and so does not serve as a historical source. It might be where your story comes from, but it might not; there may be some legitimate historical grounding. GRRM is known for his works drawing on historical examples; the AskHistorians FAQ has much on this. Hence, my comment is more a follow-up question and not a proper answer:
Did this example from ASOIAF draw on any historical evidence?