From A Wiki of Ice and Fire
The Nessaria was a merchant cog during the Dance of the Dragons.
While returning from Pentos to Volantis, the Nessaria was battered by a storm and driven off course to Dragonstone. Its sailors witnessed Sunfyre fighting Grey Ghost near the Dragonmont, and the tale soon spread throughout the island. Remembering the golden scales of Sunfyre, Tom Tangletongue led a group in search of Aegon II Targaryen's dragon.