Born in 114 AC at Hull on Driftmark to Marilda, daughter of a shipwright, Addam was raised on the docks alongside his younger brother Alyn and went to sea on his mother's trading cog Mouse. His mother claimed both boys had been fathered by Ser Laenor Velaryon, the Sea Snake's only son, though Laenor's open preference for the company of men made the claim a matter for whispers in the wineshops of the port; the court fool Mushroom said in his Testimony that it had been Lord Corlys himself who fathered them, and put them aside in Hull to keep the peace with his fiery wife Princess Rhaenys. Either way, Addam and Alyn grew up small and quick like their mother, with the silver hair and purple eyes of old Valyria, and no man who looked upon them doubted the dragon's blood that was in them.
When Prince Jacaerys Velaryon called for any of dragon-blood to come to Dragonstone and try to claim one of the wild and riderless dragons in what later was called the Sowing of the Seeds, Addam was among the first to come forward, and the wild Seasmoke, riderless since Laenor's death, lowered his head to him without resistance. At Lord Corlys's prompting Queen Rhaenyra legitimized Addam and Alyn as trueborn Velaryons and named Addam the Sea Snake's heir to Driftmark in place of her own slain Velaryon sons, and Ser Addam Velaryon flew Seasmoke for the black cause through the war that followed. After the First Battle of Tumbleton, however, two other dragonseeds the queen had raised up — Hugh Hammer on Vermithor and Ulf the White on Silverwing — turned upon their own host in the middle of the fight and gave the town over to the greens. Rhaenyra's faith in any dragonseed broke that night, and she sent Ser Luthor Largent to arrest Addam in the Dragonpit.
Warned in time by Lord Corlys, who was beaten and thrown in a black cell for the warning, Addam fled the city on Seasmoke before the queen's men could close upon him, and flew far and fast to do at his own hand what would clear his name. He came down upon castles great and small whose lords were yet loyal to Rhaenyra and gathered near four thousand men at his back, and at the head of that scratch host he fell upon the camp of the greens at Tumbleton in the dark of night. By then Hugh Hammer and Prince Daeron the Daring were already dead and their dragons Vermithor and Tessarion ran riderless among the burning tents; Seasmoke rose to meet Tessarion in the air, and the three young dragons came at last to ground amid the mud and the smoke of the burning camp, where they tore at one another until all three lay dead. Addam was thrown from his saddle in the fall and died upon the field, his loyalty paid for in his own blood. In 138 AC his brother Alyn, by then Lord of the Tides, had Addam's bones brought back from Raventree Hall to Driftmark and set upon the tomb only the single word LOYAL.