Well, I suppose you mean an eight-point cross, symbol of the Knights of St. John of Jerusalem, Kinights Hospitaliers or Knights of Malta :
They had a close relationship with the various Iberian Kingdoms and also with the Spanish Monarchy later on, it wouldn't be strange to find this symbol in a Spanish blade. But the Spanish Military Orders are different orders (founded in different places, by different people and for different reasons) and have different symbology, I'm afraid. The most important ones that were still around at the 18th. c. (and still are, nowadays) are the following ones:
Order of Santiago:
Order of Calatrava:
Order of Alcántara:
and Order of Montesa:
There's some more, but those are the most usual.