After the armies of Their Most Catholic Majesties King Ferdinand and Queen Isabella took the Iberian penninsula back from the Moors, new laws were created to unite the various Iberian governments under the Spainish crown and rid Spain of all non-Catholics. These laws provided that all non-Catholics were to be either assimilated, or banished, or killed outright. Those non-Catholic Spaniards who were of use to the crown were often given a chance to assume christian surnames and convert to the one state-sanctioned Church. Other than those who were wealthy, or integral to industries important to the survival of the state, or working in the King’s court as advisors, this meant bonded slavery under a wealthy catholic Spaniard. Even though these families converted to Catholicism they were still suspect.
The Maranos, or Jewish converts, were a favorite target of Tomas de Torquemada, the Prosecutor of the Inquisition. One of his favorite tricks to root out insincere Maranos was to stand on a hillside overlooking a village on the Jewish Sabath and mark down those homes that had no smoke rising from their chimneys. He would then ride in with his soldiers and burn those homes to the ground along with their inhabitants.
Fervent cleansing of Spain and her colonies went on for centuries, with especially rabid periods occuring under Juanita the Mad, Carlos V, and Philip II . The Spanish Inquisition wasn’t officially brought to an end until the late 1800s.
I think if you google some of the names and terms used above you will find enough fodder to keep you busy researching Spaniard slaves for quite a while.