Aegon the Conqueror's father, Aerion Targ, had ambitions like any other man. Although he loved his wife Valaena Velaryon, he fell more than once into the arms of other women. These shameful unions gave birth to two bastards that we would call today dragon seed: Orys Baratheon and you.
Your half-brother, a fellow bastard, was lucky to be without any Targ physical characteristics, while you had all the Valeryen features : silver hair and purple eyes.
From a young age, Orys and Aegon were good friends as much the Baratheon became his champion. Compared to boys, you immersed yourself in Valeryen magic, trying to learn the mystical arts practiced in Ancient Valeria on your own.
So when in -2 BC, Aegon and his sister-wives Visenya and Rhaenys began the conquest of Westeros with Orys Baratheon, you stayed on Dragonstone to study. The people of the island began to avoid you as you learned more sorcery. Rightfully so, they were afraid of your abilities.
A few years after the beginning of the reign of Aegon the Conqueror and his sister-wives Queens Visenya and Rhaenys, your step-brother came to visit you late at night in your room. He lived half the year on Dragonstone but rarely spoke to you.
“I had a dream,” Aegon told you, his face serious as he approached you. “and I will need your help to decipher it.”