I'm reading GRRM's Fire & Blood and hearing about how the entire Gardener family was extinguished in a single battle struck me as odd. Why was no one left at behind to continue the line just in case? Did this happen in real life as well?
Maybe not exactly what you are looking for because it focuses on one death, but that death ended two centuries of rule by the House of Aviz over Portugal and resulted in the kingdom being taken over by Spain.
In 1578, Sebastian I of Portugal went on a crusade to Morocco, with the intention of re-installing Abu Abdullah Mohammed II as a friendly sultan on the Moroccan throne. Abdullah Mohammed II had been usurped by his own uncle, Abd Al-Malik I, two years before, after his uncle had convinced the Ottoman Empire to help him take the throne. Sebastian I was twenty-four years old, and he had been king his entire life (most of it under regents) as his father died before he was born. Although negotiations were under way to find him a wife (either from Spain, France, or Austria) he was still unmarried and childless. Yet, eager to prove himself in war, and seeing an opportunity to push back the Ottomans, he gathered an army and fleet and set off to join forces with Abdullah Mohammed.
So the two kings bravely marched into the interior at the head of an army of about 24,000, which included the flower of the Portuguese nobility, mercenaries from Spain, Germany, and Italy, and several thousand Moors under the deposed Sultan. Unfortunately for Sebastian, Abd Al-Malik (who was gravely ill at the time) had mustered an army that was at least twice as numerous, with a large force of cavalry.
The Battle of Alcacer Quibir went somewhat predictably. The Moroccan force deployed in a wide formation, and its cavalry quickly enveloped the Portuguese army. Sebastian led a charge into the heart of the Moroccan army but was cut off and never seen alive again. Abdullah Mohammed drowned trying to escape across the Loukkos River. A third of the Portuguese army was killed, and virtually all of the rest were captured. The only real surprise was that Abd Al-Malik also died during the battle, but that was because he had decided to lead his army from horseback despite his illness- so the third king to fall in the battle died of natural causes.
Portugal was thus bereft of its youthful king and a significant part of the nobility. The crown went to Sebastian's great-uncle Henry, as he was the only clear successor- but Henry was an elderly Cardinal and had lived a celibate and thus childless life. Henry himself died two years later. Philip II of Spain, due to intermarriages between the houses of Habsburg and Aviz, was both Sebastian's uncle and Henry's nephew- and thus had about as strong a claim to the empty throne of Portugal as anyone else. Of course, it also helped that he had a much larger army than any other claimant. He sent the Duke of Alba with an army to claim Portugal, which was in no shape to defeat the invasion- and most of noble families, devastated by losses and ransom payments, decided to support Philip II anyway.
It's important to remember that between child mortality, disease, battle, and good 'ol infertility, noble families died out all the time. And being the last male hope to continue a line did not generally excuse one from responsibilities to go into battle. Historically, childless heads of a family, or perhaps a father and a couple adult sons certainly died together in battle, at least occasionally. But to have potentially dozens of males from the same family all go into battle together to die, as in the case of the Gardeners at the Field of Fire, is to take a fair bit of literary license. GRRM is telling us that Mern IX of Highgarden, out of all his sons, grandson, cousins, nephews, etc.- they were all of military age and all happened to get killed in the same battle? Not one ran away when dragons showed up, or had a little boy waiting at home? It's just a convenient way of explaining how the Tyrells come to power in the Reach.
In Japan, at the end of the Genpei War in 1185 CE, the Taira clan (often referred to as "Heike") were essentially wiped out after the sea battle of Dan no Ura ( 壇ノ浦 ). The story is told in the celebrated Heike Monogatari; and it seems to be roughly true in the historical bits. Another account, the Azuma Kagami, also has some useful information, but is similarly a mix of fact and fiction. The Taira, a dominant force before 1185 essentially disappear from the scene after Dan no Ura.
The Taira Emperor Antoku (aged 6 or 8) and the leaders of the clan either died in the battle, were executed, or committed suicide by jumping into the sea. The story has a lot of evocative elaboration-- it's said that the great sword of the Imperial Regalia, "Kusanagi" (草薙劍) was lost in this battle, disappearing beneath the waves with the young Emperor; rumor has it that the one in the Atsuta shrine at Nagoya is a copy! Another local folk tale is that the souls of the Taira warriors are carried in the local crabs, called "Heikegani" who crawl the ocean floor, looking for the lost Imperial treasures. Antoku himself is sometimes thought to be a Shinto water deity, worshipped as "Mizu no Kami" (水の神 = "God of Water" ).
The young Emperor's soul was apparently an unquiet spirit, so a few years' after the battle efforts had to be made to placate it:
In commemoration of this event, Amidaji, a temple that overlooked this sea, was revived as a mortuary temple for the drowned emperor and Taira clansmen. Because the tragic deaths of the child emperor and the Taira were followed by various calamities, it was believed that their spirits, unable to access the next world, became malicious ghosts who threatened the living and the nation. In order to appease these ghosts and to assist them in attaining rebirth in the Western Paradise of Amida Buddha, the imperial court ordered Nagato province to construct a Buddhist hall at Amidaji in 1191
One tantalizing suggestion with a reasonable historical basis is that they didn't all die, but rather like the Trojans in the Aeneid, fled across the sea, at least one scholar suggests that they may have ended up in the Ryukyu Islands. Japan also has a tradition of "Heike-dani", "Heike valleys", where the defeated clan might have lived on. Leaving a known heir someplace visible wouldn't have worked-- because the victorious Minamoto killed all the Taira they could find-- so to the extent that survival was possible, it had to be hidden.
Whatever the case of this covert survival, as a clan on the throne or near it, the Taira disappear after 1185.
Sources
Local Legends of the Genpei War: Reflections of Mediaeval Japanese History
The Heike: from Defeat at Dannoura to a Golden Age in Ryukyu?
The Heike monogatari and The Japanese Warrior Ethic