To unlock it, you must occupy the bonfire that you have in your inventory at the beginning of the game, reach the round where the conquerors appear, what I recommend next is to let two cities expand, the capital always with protection, what follows next It is to wait turn by turn for the red event that I have as an image on the cover to appear, what you should see in the mission is that the "Chichimeca boss" is found, the sketch as a reference you can see it in the "arena" position 48.