The Aztec priest and adviser who reformed the empire was Tlacaelel. He served as cihuacoatl, or chief adviser, during the reigns of several Mexica rulers and became one of the principal architects of imperial ideology.
Tlacaelel was the nephew of Itzcoatl and the brother of Chimalpopoca and Moctezuma I. After the Mexica defeated the Tepanecs in the late 1420s, he helped shape the Triple Alliance between Tenochtitlan, Texcoco, and Tlacopan. This alliance became the political foundation of the Aztec Empire.
His reforms were religious, political, and historical. Tlacaelel elevated Huitzilopochtli, the Mexica tribal deity, to a central position, promoted military expansion, and strengthened the role of human sacrifice in state religion. Later accounts also credit him with encouraging the Flower Wars and with ordering historical traditions to be revised or destroyed so the Mexica could present themselves as a chosen people.
Tlacaelel is sometimes confused with an emperor because of his enormous influence, but he never ruled as tlatoani. His power came from advising rulers and directing imperial policy from behind the throne.