Princess Alysanne Targaryen and Ser Walter Tully spent the early years of their marriage in relative peace. They brought forth no children, instead doting upon Ser Walter's growing nephews as if they were their own. Being brought up in the Red Keep alongside Rhaenyra and Aegon had taught the princess what problems that can arise when succession becomes unclear. Any children that she and the future Lord Tully bore would be destined to fight their cousins for the seat at Riverrun. Lord Grover Tully was somewhat vocal about his displeasure that the princess had not provided him any great-grandchildren that could grow up to be dragonriders, but he was an old man and increasingly bedridden from illness.
The princess' own father, King Viserys, was similarly in decline. Though it was well-known that the King had spent most of his life fighting a disease that the maesters did not fully understand, the extent of his suffering was hidden by those among him, especially once Princess Rhaenyra returned to Dragonstone after the tragic death of her husband, Ser Laenor. When Rhaenyra left to be with her sons, Princess Alysanne considered flying to King's Landing in her stead. Though the Hightower branch of her family loved her dearly, she did not trust them to report on the King's state honestly. Her loyalty had always been with her sister, whom they were growing increasingly aggressive toward in recent days. But Aegon's threats lingered over Riverrun and as the prince aged, he had only grown more cunning and cruel.
In a decision she would later come to regret, Princess Alysanne chose her safety and happiness over the comfort of her dying father and the loyalty she owed her older sister. Mere months after Rhaenyra flew to Dragonstone to take charge of her sons, King Viserys died and Aegon Targaryen was annointed King Aegon, second of his name, in the dragonpit on the order of his grandsire, the Lord Hand, and his mother, the Queen. Queen Alicent's sworn protector, Ser Criston Cole, placed the Conqueror's crown on Aegon's head in front of a crowd of thousands.
A day later, a second coronation and a wedding followed on the island of Dragonstone. Princess Rhaenyra was crowned queen and wed her uncle, Prince Daemon whose wife Laena Velaryon had recently died in childbirth, in a Valyrian ceremony. The marriage was deemed distasteful by many in the realm as the two were quite far apart in age and neither of their previous spouses had been dead for more than six months. Some among their enemies even went so far as to accuse them of having in hand in Laenor's death from sparring injuries.
Though she had hesitated to leave Riverrun before, remaining passive would've been folly for Alysanne Targaryen. By naming Aegon king, the Hightowers had given him authority and permission to act as he pleased with impunity. Her brother had never had interest in ruling. He'd never shown much interest in anything, aside from her. Aegon would have only one goal to aim his new resources and powers towards accomplishing.
It was not long after news of the coronations reached Riverrun that Princess Alysanne and Ser Walter announced their intentions to fly for Dragonstone with their nephews. Silverwing had never before borne four riders at once, but it was essential that the boys be moved to safety both for their own sakes and for the future of House Tully. Lord Grover Tully, a proud man and ever a contrarian, forbade them from leaving. Having Alysanne and Silverwing at Riverrun made House Tully a player in the war that was to come and he did not want to lose the influence he currently held. Lord Tully misunderstood his situation, however. Princess Alysanne may have been a member of his household but as a dragonrider and a beloved princess of the realm, there was little he could do to control her. Many of the house guards felt hers was a higher authority than his own, both legally and morally. Lord Grover Tully had not fostered love among his men, most of his companions were long dead. But his grandson had grown up training amongst the men who now manned the castle. Ser Walter, Princess Alysanne, and their nephews faced no resistance when they fled Riverrun for Dragonstone, but rather were helped with their packing and escape by a myriad of servants and guards.
Alysanne Targaryen and her companions were greeted warmly by her sister upon their landing at Dragonstone. Any infighting between members of House Targaryen was likely to end in war between dragons and, though few spoke of that reality, all of Rhaenyra's allies were glad to have another dragon added to their cause. Silverwing was not as experienced in combat as Caraxes or Vhagar but she was over a hundred years old and quite large. She had faced Sunfyre already and, though it had not come to combat, been a serious enough threat to cow Prince Aegon.
Perhaps more important to their cause than Silverwing, was Alysanne herself. In Alysanne, Rhaenyra's cause had gained indisputable leverage over Aegon, though her small council disagreed on how to use that leverage. All agreed that she could make a valuable diplomat for dealing with Aegon and his supporters. Some whispered that perhaps the best thing Alysanne could do for their cause was to give herself to Aegon. If he had her, he might withdraw his claim to the throne. These whisperers were often minor lords and knights from outside the Crownlands. No one who had known either Alysanne or Aegon would think this a fair or useful idea. Those who knew the princess loved her too dearly to send her to such a fate. Those who were familiar with the claimant king knew that his pride had been wounded at Alysanne's wedding and the powerlessness he'd felt that day was something he'd fight fiercely to avoid.
Princess Rhaenys Targaryen and her Lord husband, Corlys Velaryon, proposed their own plan for how to use the princess. If Alysanne was spotted anywhere with defenses weak enough that Sunfyre alone could win a fight, he would come for her, even against the advice of his grandsire and mother. The Velaryons had made their living from the sea for generations, they knew how to spot and use bait. Though Princess Alysanne supported the plan, it was struck down by the Queen. She would not risk the safety of her beloved sister even for a shot at Aegon. The best thing to do, as war seemed inevitable but had not yet arrived, was to gather their allies.
Dragonstone, as an island, was highly defensible but a difficult place from which to field a sizeable army. So, it was decided that Rhaenyra's cause would need to find a stronghold somewhere in the center of Westeros for her loyal lords to gather their armies at. Ser Walter Tully offered Riverrun. His grandsire had desired influence over the coming war and would likely give his consent. Riverrun, however, was determined to be too far from King's Landing to be an ideal tactical position. Queen Rhaenyra did, however, note Ser Walter's offer and say that if they were able to recruit the Northmen to their cause, Riverrun could prove an important stop on their journey south to join the fighting.
Harrenhal, which was much larger than Riverrun and held a better location, was the favorite of Rhaenyra's council. Lord Larys Strong, who possessed Black Harren's keep, was, however, one of Aegon's strongest supporters and served as his Master of Whispers and Lord Confessor. To Prince Daemon Targaryen, this made Harrenhal doubly appealing, as taking it would be a blow to Aegon's cause.
With the Queen's consent, Prince Daemon flew to Harrenhal and took it without bloodshed. Harren's keep had faced dragonfire before and it's castellan, Ser Simon Strong, had no desire to share Black Harren's fate. With Harrenhal secured as a base of operations, Queen Rhaenyra sent for her supporters. The lords of her small council and those who had already sworn to her a second time ordered their men to march to Harrenhal, but the host lacked in numbers. The Queen would need strong allies to fill in the gaps of her army.
At their own request, the Princes Jacerys and Lucerys and the Princess Alysanne were sent as messengers to the Great Houses that were likely to support Queen Rhaenyra's cause. Alysanne flew to Riverrun to treat with her husband's grandsire. Jacerys rode Vermax north to the Eyrie and Winterfell. Prince Lucerys, whose dragon, Arrax, was too young to manage a long flight, was sent on the short flight to Storm's End to request the aid of Lord Borros Baratheon and to remind him of the oath his father gave to Queen Rhaenyra when she was named heir by King Viserys.
Unfortunately, Prince Lucerys was not the only dragonrider sent to Storm's End. His uncle, Prince Aemond, had arrived before him and had offered Lord Borros, who had four young daughters, a marriage pact in exchange for the support of his house. Prince Aemond was an untested but long-practiced fighter who was ruled by an overwhelming sense of duty. He was eager to prove himself and possessed the Targaryen fire that had hindered and aided many of his ancestors.
It is unknown what exactly happened in the skies above Shipbreaker Bay or how a fight broke out between the two princes and their dragons. But given Prince Aemond's temperament and the vile nature of the taunts he levied at his young nephew while under Lord Borros' roof, most historians are inclined to believe that he was the one to begin it. Perhaps the clearest point of evidence we have for saying Prince Lucerys did not desire a fight is the size of their respective dragons. Lucerys rode Arrax, whose egg had been placed in his cradle and who had only become large enough to bear the prince's weight in recent years. Prince Aemond's mount, the mighty and ancient Vhagar, was the living largest dragon in the world. Prince Lucerys was no fool, he would have known that any conflict between the two dragons would leave him egregiously outmatched.
In the end, a fight did happen and young Prince Lucerys Velaryon lost. His body and that of Arrax washed up on the shore of Griffin's Roost three days later. Both had been snapped in half by a large set of dragon teeth.
Princess Alysanne was at Riverrun when her nephew died and only learned of the tragedy upon her return to Dragonstone. Her own trip had been much more successful. Lord Tully was sending a host of men to Harrenhal. This victory, though, seemed insignificant when faced with such a loss. Princess Alysanne had no heart for violence and she was horrified that her younger brother whom she'd had traveled the nation with had turned out to be capable of such cruelty.