Answer:
The Conquistadors were carriers of — diseases. Because the source of the diseases could not be seen with the naked eyes, they were labelled Invisible Warrior.
The diseases included but were not limited to
Smallpox, measles, mumps, typhus etc. Because the Native Americans had never come in contact with these ailments before, they neither had the cure not the immunity against them.
The diseases ended up killing them by the hundreds of thousands.
Cheers.