The Battle of Wayna Daga (Amharic for "Grape-cultivating altitude") occurred 21 February 1543 east of Lake Tana in Ethiopia. Led by the Emperor Galawdewos, the combined army of Ethiopian and Portuguese troops defeated the Muslim army led by Imam Ahmad ibn Ibrihim al-Ghazi. Tradition states that Ahmad was killed by a Portuguese musketeer, who had charged alone into the Muslim lines. Once his soldiers learned of the Imam's death, they fled the battlefield.
Once the Ethiopian-Portuguese army found the army of Imam Ahmad, they set up camp nearby; Emperor Gelawedewos advised against engaging the enemy right away, hoping that the 50 missing Portuguese soldiers would arrive soon; "in that country fifty Portuguese are a greater reinforcement than one thousand natives." Over the following days, each camp preceded to harass the other with cavalry raids. The allied side had the better of the exchange, keeping their opponents from venturing from their camp for supplies, until the Muslims resorted to a stratagem to kill the leading Ethiopian soldier, Azmach Keflo, which demoralized the Ethiopian troops. Faced with the potential desertion of his force, Galawedewos decided he could wait no longer and prepared for an assault the next day.
The two forces started the main battle early the next day, with the Muslim force divided into two groups. At first, the Muslim side succeeded in driving the allied side back, until a charge by the Portuguese and Ethiopian cavalries broke up the charge. At this point Imam Ahmad ibn Ibrihim al-Ghazi, with his son at his side, took to the field and led a renewed assault. It was in this charge that the Imam was killed by a bullet to his chest which threw him from his horse, although the sources differ in how he died.
According to Castanhoso, the Imam was recognized by the Portuguese arquebusiers, who directed their combined firepower at him, and one of the arquebuses in the group fired the fatal shot. Although he was an eyewitness to the battle, Castanhoso constantly emphasizes in his account the corporate identity of the Portuguese expedition after Gama's death: "We bore before us the banner of Holy Compassion (Sancta Misericordia); the Preste had sought to appoint one of us Captain, but we desired none save the banner of himself to lead us, for it was not to be anticipated that we should follow another, having lost what we had lost."
There is another tradition, at least as old as João Bermudes, and repeated by every other near-contemporary source (e.g., Gaspar Correia, Jerónimo Lobo), that gives the credit of Imam Ahmad's death to one João de Castilho; João charged into the Muslim troops so he could fire upon Ahmad Gragn at point-blank range, an audacious act resulting in his death. Both Castanhoso and the story of João de Castilho return to agreement about Imam Ahmad's fate after this point: at the end of the battle, when Emperor Galawedewos offered his sister's hand in marriage to the man who killed the Imam, an Ethiopian soldier presented the Imam's head as proof of the deed; but a subsequent investigation revealed that the Portuguese had wounded him before the soldier had cut off the Imam's head, "thus he did not give his sister to that man, nor did he reward the Portuguese, as it was not known who wounded him"