A Storm of Swords
Daenerys Targaryen travels to Astapor to view the Unsullied. The slaver Kraznys mo Nakloz has his translator, Missandei, tell Daenerys that three days earlier a corsair king was interested in purchasing a thousand of the troops. Missandei corrects the Good Master, and says that the unnamed corsair was interested in only one hundred Unsullied.
A Feast for Crows
It has been theorized by some readers that the corsair king is actually Euron Greyjoy using an alias.