Which happened as a personal union in the person of their grandson, HRE Charles V, and as a merged nation-state (note that this concept didn’t even exist at the time) in that of Felipe V, who merged the crown of Aragon into that of Castille.
While Isabel didn’t kill Fernando to get the throne of Castille, she was the winner of a civil war between her side and that of Juana la Beltraneja, who was officially the daughter of Enrique II “the Impotent” of Castille and of his second wife. The nickname of “la Beltraneja” is an accusation that she was actually the daughter of the king’s closest friend, Beltrán de la Cueva. Enrique was the uncle of Isabel and step-brother-in-law-by-first-marriage of Fernando (the Trastamara’s family tree is more of a bloody mess).
Juana II Evreux and her husband, Felipe III, were co-regnants of Navarre.
Some of the ladies of the Court of Toledo spent as much time trying to replace the current King (whomever it happened to be) as the lords, but the ladies couldn’t be elected king. I don’t remember specific names right now and the books where I read about the ladies in question are at my mother’s house.
Of king “consort” like Darnley, he was neither his wife’s heir nor her co-regnant. But so were Isabel and Fernando for each other, Blanca I of Navarre and Juan I of Aragon for each other… each reigned in his/her realm and was consort of the other one.
Dowager Empress of China, Tzu-hsi (Cixi) was made co-regent when her six year old son inherited the throne in 1861. By the time the boy Tongzhi succeeded to the throne at age 17 he had been indulged, with the complicity of his mother, to the extent he was addicted to opium and died from venereal disease aged 19 in 1875. Tzu-hsi then installed her 3 year old nephew Guangxu as emperor with herself as co-regent.
After Guangxu succeeded to the throne in 1889 Tzu-hsi remained the power behind the throne for 9 years before resuming control in a coup and placing the emperor under effective house arrest. The last decade of her life was tumultuous, including the Boxer Rebellion and in 1908 Tzu-hsi had Guangxu poisoned before dying herself the next day having ruled China for nearly 50 years.
Margaret of Anjou, the wife of Henry Vi pretty much ran the show, because her husband wasn’t really capable of ruling.
She remained very loyal to him, but she was the one who was in charge of the Lancastrian side during the Wars of the Roses. The Yorkist Edward IV said that he feared her more than all the Lancanstrian lords together, and Shakespeare called her ‘a tiger in a woman’s skin’.
Technically, although she was born in Prussia, she wasn’t actually Prussian; she was a princess of Anhalt-Zerbst, a rival principality within the Holy Roman Empire. Her brother, Frederick Augustus, was the last prince of Anhalt-Zerbst, and was forced into exile when the Prussians invaded his lands. When he died childless, the principality was eventually divided between the related houses of Anhalt-Bernburg, Anhalt-Dessau, and Anhalt-Köthen, which themselves eventually merged to form the Duchy of Anhalt. The Duchy became part of the German Empire and later republic, but remained separate from Prussia until after WWII.
Mary I of England married Philip II of Spain, and he was given the title of King of England and Ireland, to last for the period of their marriage (in addition to the titles of King of Naples and Jerusalem that his father had already given him!), but I confess to not knowing to what extent he really ruled with her. They were both equally devoted to the idea of restoring Catholicism in England, so it probably made no great difference for the relatively short period of her reign, and Philip bowed out when Mary died having named Elizabeth as her heir, according to their father’s wish.
Trying to get an accurate fix on the Egyptian Queen/Pharoah Hapshepsut (18th Dynasty, circa 1475 B.C) is difficult. She was the daughter of the Pharoah Thutmose I and his main wife and she married her half brother Pharoah Thutmose I, whose mother was a secondary wife of a Thutmose I. They had a daughter but no sons. Thutmose II had a son, Thutmose III (the family was as original with names as the Barrymores) with a secondary wife. When Thutmose II died, Hapshepsut became regent and a few years later became Pharoah, confusing the sculptors and inscription writers who gave her a mix of male and female symbols. She sent an expedition to a place called Punt (probably Somalia) and in the temple dedicated to her, her stepson is shown merely burning incense. Tuthmose III later proved in his military campaigns in Palestine that he was very capable, so exactly how Hapshetsut ran things and how long is argued about.
Roman Emperor Claudius (40-54 AD) was almost overthrown by his wife Messalina (various Roman historians and writers such as Juvenal have nothing good to say about her; they may have an axe to grind since apparently a source were writings by Nero’s mother Aggrippina, who moved Nero past Messalina’s son Britainnicus in the succession ). But Claudius’s staff, several freed slaves, were able to get him to crush the coup she was planning with a consul, and when they had Claudius liquored up, got him to sign a death warrant for Messalina.
Cleopatra VII (the famous one) was married to her younger brother (by eight years) Plotemy XIII and they were co rulers. Various infighting ensued and eventually Cleopatra got help from Julius Caesar to overthrow and kill Plotemy XIII. She then married another still younger brother Plotemy XIV who had the sense to realize who ran things and stayed in the shadows, until apparently Cleopatra poisoned him shortly after Caesar’s murder.
Livia Drusilla was the wife of Gaius Octavius (the emperor August) for 51 years including 40 as emperor. Accounts of the period vary widely depending on whose axe was being ground but for most of that time they could be considered co-rulers and that Livia poisoned Augustus so as to continue as the power behind the throne for her son Tiberius.
The part of the I Claudius novels about Livia poisoning Augustus is supported by various other sources so my post was accurate regardless of what cites I included.
Sidetrack: while fictitious, I think it is a stretch to refer to Robert Graves’ writing as pulp fiction. He did do significant research and does not deviate from accepted fact.