He asking the expedition to a silence a commander by the name of Coyotl, who is continuously asking the merchants to give corns and seeds to the people. If the merchants would give it away, they wouldn't be able to business for long. The commander is getting more persistent and even threatens the merchants with force to hand over the goods.
As such, the merchants are glad to take him out of the picture and since the commander is despised among his troops, they believe that taking has little consequences and his successor would stop making demands to hand over food. They need his shield as proof of the deed being done.
From the market, head east over the dam and then follow the path to the north until the expedition comes across a town with a market. There the commander can be found. You can either open the attack immediately after meeting him or go through all dialogue to open the option of acquiring food and attacking the merchants
This goal keeps all options open until the player either attacks Coyotl and his warriors or the merchants.
Acquiring 300 rationsEdit
The best way to acquire large amounts of rations is by trading with the cities of Papantla northwest and Xalapa in the north, while Tenochtitlan can also be considered. Rations can also be bought at the farm along the road to these northern cities, but prices are expensive. Although hunting for pigs in Mexico can bring in some rations, it's not the most reliable source of food. That method should be considered if exploring the lands is the main goal.
Then there is also a civilian with two oxen who travels between the cities near the lakes. He can be pursuaded two sell one of his oxen, but he can be attacked for both them, which will give 30 or 60 rations respectively. By giving him 375 valuables for one ox, he is willing to sell them both to the expedition.
If the player needs some achievements like Tactician, Of Pens and Sword and Keep your Distance, this battle will provide an easy method to earn those.
After acquiring more than 300 rations by various methods, bring them to Coyotl, who will load them into the warehouses with his men. He then gives his shield willingly. Then head for the market of Tenochtitlan for the reward.