Which Aztec emperor was known as 'Obsidian Serpent'?

The Aztec emperor known as “Obsidian Serpent” was Itzcoatl. His Nahuatl name combines itztli, meaning “obsidian,” with coatl, meaning “serpent” or “snake.”

Itzcoatl ruled Tenochtitlan from 1427 to 1440 as its fourth tlatoani, or ruler. He came to power during a crisis after the death of his predecessor and joined forces with Nezahualcoyotl of Texcoco and the ruler of Tlacopan. Together they defeated the Tepanec power centered at Azcapotzalco and established the Triple Alliance, the political foundation of the empire later called the Aztec Empire.

Under Itzcoatl, Tenochtitlan expanded its control over important agricultural and lake regions, including areas around Xochimilco and Chalco. He is also associated with major changes to the empire’s historical and religious presentation, although details come mainly through later sources that may reflect political reshaping of the past. The title is sometimes confused with other Aztec rulers, but Itzcoatl is the emperor whose name itself means “Obsidian Serpent.”