In order to write a good introduction, you must first have some idea as to what you want to argue in your essay. For me, the most important thing about the Incas at this time is that they were about to be conquered by a very few Spanish conquistadores. I would want to explain why that was possible. My essay and my introduction would focus on that topic. My introduction would set out what I thought were the most important facts about the Incas that would explain why they would soon be conquered.
I would point out that the Incas had a vast empire that they gained through conquest. I would point out that this made them have many enemies within their empire who were waiting for a chance to rebel. I would point out that the Incas had no immunity to the diseases the Spaniards were introducing to South America. I would further point out that the Incas did not have a set way of deciding who the next emperor would be when one died. I would then put that together by saying that, just before the Spanish came, an epidemic killed many Incas, including the emperor. A civil war followed to determine who would be the next emperor. Then the Spaniards came. Using other Indians who hated the Incas as help, they destroyed the Inca empire.
In this way, my introduction would briefly present the major factors that led to the fall of the Inca Empire.