Oh, for heaven's sake. As noted in the second sentence of
the wiki article about Droit de seigneur (the French equivalent of Latin
ius primae noctis):
There is no historical evidence that such a right ever existed.
The article goes on to explain that the widespread belief in such a law may have started with the Greek historian Herodotus, who claimed it was an exotic custom of a "barbarian" tribe in Libya. (In other words, Herodotus was quite possibly repeating a "those wacky foreigners" Urban Legend he'd heard secondhand from Greek travelers.) Much later, Voltaire popularized the concept as part of his satires against the ruling classes, and people took Voltaire as gospel.
P.S. In Latin,
noctum is the genitive plural form of
nox (night). So
prima noctum is ungrammatical, but you could say
primarum noctum, which equals
первых ночей ("of the first nights"). Or, better yet, just say "the alleged right of a king/lord to deflower his subordinates' brides", in plain English.