Several key elements of the Empire's history play a major part in the story, including the Triarchy of the title, a triad of priest-kings whose forerunners once ruled a great kingdom, but who are now essentially a terrorist organisation. And even more prominent is one of the main characters: the dowager empress Novesh, her own brother's widow.
A crucial aspect of the empire was that, as the early warrior-emperors (and empresses) gave way to heirs with no special talent, the attitude grew that the emperors were not only too sacred to marry anyone but their siblings, they were also too sacred to sully themselves with the dirty business of ruling the empire. Which meant, of course, that power was wielded by a variety of ambitious and corrupt ministers and generals.
Royal sibling marriage isn't unknown in real-world history, most famously in Egypt and the Incan Empire. Since the latter left no written records, we know of its customs mainly from the highly biased accounts of the conquistadors and the somewhat-more-reliable oral traditions passed down to the descendents of the Incas' subjects. In the circumstances, we can largely only assume their reasons.
The Egyptians certainly left written records, but these don't always tell us what we want to know, since they were often written for the purpose of propaganda, not of history. It can be difficult, for instance, to be sure just how many Pharaohs actually married their sisters, since the consort was automatically referred to as his sister, even in cases where Egyptologists are fairly sure that she was unrelated.
We can be fairly sure that the main reason (or maybe the excuse) for the practice in both civilisations was, as in the Sheballan Empire, that the ruler was divine and consequently could only marry someone equally divine. Many mythologies portray the gods as marrying incestuously, including the Greek god Zeus, who married his sister Hera and had a child with another sister, Demeter.
Was that the only reason, though? Another plausible motive was to avoid having annoying in-laws — and, if that sounds like a rather weak joke, you only have to think how often in history a consort's family has constituted a major challenge to the royal family.
Two of Henry VIII's wives, for instance, came from the Howard family, whose head, the Duke of Norfolk, was one of the most powerful men in the kingdom — and would probably have been even more powerful if both Howard wives hadn't lost their heads. Jane Seymour gave Henry his heir and conveniently died of the birth, and her brothers virtually ruled the kingdom after Henry's death. Today, with a constitutional monarchy, the Spencers are nothing more than a mild embarrassment to the Windsors, but five hundred years ago they might have been raising rebellion to protect the rights of Diana's children.
If we don't know enough about the Incas' motives and practices, or even the Egyptians', there's rather more information about the most famous dynasty in western history to practice sibling marriage: the Ptolemies, who began with Ptolemy Soter, general and reputed half-brother of Alexander the Great, and finished (barring a six-month successor) with Cleopatra VII, lover of Caesar, wife of Antony and seductress of history ever since.
The Ptolemies ruled in Egypt but, like Alexander, they were Macedonian Greeks. It's been speculated that there may have been some Egyptian or other African blood in the dynasty by the end — that's not impossible, but unlikely, and there's certainly no evidence for it. The only non-Macedonian blood known to have flowed in Cleopatra's veins was actually Persian.
Despite their mythology, the Greeks — and the Macedonians claimed to be Greek, even if they weren't always welcome members of the family — had a particular horror of incest. True, their main disgust was aimed at mother-son relationships, such as Oedipus's, but any hint of incest was distinctly non-Greek. So what possessed Ptolemy's family to adopt it so enthusiastically?
Well, for one thing they were in Egypt. Egyptian culture fascinated the Greeks, who regarded them as a wise and ancient people, even while also regarding them as just plain weird. Although the Macedonian rulers and the Egyptian subjects kept largely separate, the Ptolemaic king was also crowned Pharaoh, to keep his subjects happy, and some of the family fell under the spell of Egypt.
The first sibling marriage in the dynasty was almost certainly a matter of convenience. Ptolemy II was an able ruler, but possibly not the strongest-willed of men, whereas his big sister Arsinoë knew exactly what she wanted. Macedonian women could be formidable, but they didn't have constitutional rights to rule in their own name — they had to marry a ruler. Arsinoë had tried this without any great success, and her brother's first wife (also called Arsinoë — the same names recur a great deal in this dynasty) had died. Perhaps she knew of old that she could make her brother do what she wanted. And this was Egypt, so there was a precedent.
The couple had no children (Arsinoë was probably past child-bearing age) but she proudly took as her cult-name Arsinoë Philadelphus: She Who Loves Her Brother. The city of Philadelphia mentioned in the bible and copied by William Penn and co was named after her. It meant that kind of brotherly love.
Ptolemy III (son of his father's first marriage) didn't marry his sister, though his wife was a first cousin, but their son revived the practice. Ptolemy IV's viciousness was somewhat tempered by his laziness and obsession with pleasure-seeking, but his favourites made up for the lack, murdering the entire royal family apart from the King's youngest sister, another Arsinoë, who was married off to him. This was certainly to do with having no inconvenient in-laws. Ptolemy's male relatives had had to go because they might have challenged the favourites, and the King's in-laws could have had similar power. Poor Arsinoë had no-one.
In fact, Arsinoë Philopator seems to have been a remarkable woman. The great scientist Eratosthenes, who successfully measured the size of the Earth, had been tutor to the royal children, and he later wrote a memoir expressing his adoration for her. Several stories of her life suggest that, like her most famous descendent, she had remarkable charisma.
That didn't help when Ptolemy died and Arsinoë tried to claim the right to rule as regent for her infant son. She was assassinated by the favourites, but the Alexandrian mob rioted at the rumour and lynched them.
Ptolemy V, then, was the son of siblings, who were the children of first cousins, but there's no sign in him of any inbreeding problems. That's not really so surprising. The child of sibling parents has a slightly higher than normal risk of congenital abnormalities, but it's still relatively low — it takes two or three generations of the practice before the threat becomes serious. This pattern may account for the reason why the various Egyptian dynasties tended to flourish for a few generations and then decline.
Since he was the only member left of the royal family, Ptolemy V married a Seleucid princess (Macedonian rulers of Syria) who brought a new name into the dynasty — Cleopatra. The next few generations, though, were rife with incestuous marriages; one king married first his sister, who had previously been married to his elder brother, and then the daughter of his two siblings.
These Ptolemies and Cleopatras — vicious and decadent, but showing no sign of debilitating conditions until perhaps the last generation — fought one another into extinction, leaving only an illegitimate child to be put on the throne. Ptolemy XI probably benefited from bringing new blood into the family, though no-one knows what blood that was. Similarly, his queen was described as his sister, but this might just have been the old Egyptian custom, rather than suggesting they were related by blood. Certainly, the most famous of their children, Cleopatra VII, appeared genuinely interested in Egyptian culture and frequently dressed up as the goddess Isis. This has affected the popular image of her appearance, although she was said to be a redhead under the black wig.
Neither the Incas, the Egyptian dynasties nor the Ptolemies sustained a run of sibling marriages for long enough to have the devastating effect on the bloodline that I've portrayed in The Triarchy's Emissary. The parallel lack of any political power there meant that having a dribbling idiot on the throne was no great disadvantage. Indeed, it could be helpful to those who exercised the true power. Novesh's brother-husband fitted that description, but Novesh herself — as can happen, even after generations of inbreeding — somehow escaped the effects. Like the Ptolemies, her family produced one last magnificent woman at the very end.
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