Towards the end of the Century of Blood Aegon Targaryen, the last trueborn scion of House Targaryen, agreed to aid Pentos and Tyrosh in their conflict against Volantis. No one quite knows what caused Aegon to agree to the request, but he kissed his sisters goodbye and mounted Balerion. He flew to Pentos to meet with the Prince of the city and the Magisters of Tyrosh, ostensibly to discuss battle plans and to determine the kind of aid he would be providing. During his stay, the night prior to leaving for Lys, Aegon had a dream.
He’d always been a dragon dreamer, all riders of Balerion were, but he had never had a dream like this. It was of Westeros, the backwards continent his family had fled to before the Doom, and a great and terrible winter gusting from the North. He dreamed of an absolute darkness that would follow and the blue-eyed beings that would destroy the world of men. He dreamed of the kingdoms of Westeros united under a dragon on a high throne defeating this darkness. There other details he missed, namely how equal the other kingdoms were and the prevalence of a certain dagger. He was certain of what the dream was telling him.
He did not write the dream down.
So Aegon is horribly injured following the death of his dragon Balerion, all thoughts of the dream are pushed from his mind. He was taken to Pentos to recover from his wounds, a thank you for his help in protecting them from Volantene aggression. It is said he raved while drugged on milk of the poppy, speaking of his dream in fragments and snippets. A healer’s assistant attempted to write it down, but only managed a few details. When Aegon eventually succumbed to his injuries, the true nature of his dream died with him.
On Dragonstone, Aegon’s sister-wives were facing a different issue.
With their brother dead, and neither of them showing signs of being with child, it was unclear who would inherit Dragonstone. Orys Baratheon, the bastard brother of Visenya and Rhaenys, attempted to make a case for his own claim in a closed meeting with his sisters. None save for the siblings know what was said, but Visenya soon declared herself Lady of Dragonstone. As her first act, she exiled her half-brother to the mainland.
Word quickly spread among the kingdoms of Westeros about the Dragon Widows, either of whom would bring the ultimate weapon to the house she married into. Kings, princes, knights and even petty lords all flocked to Dragonstone to try and claim a Dragon Widow.