Nahualli is a term which denotes two different, yet connected things. In one use, the word nahualli is the term used for a spiritual twin of a person, almost always an animal form. The gods have nahualli as well. For example, Tezcatlipoca’s nahualli form is Tepeyollotl, a jaguar god, and Huitzilopochtli’s are the hummingbird and the eagle.

Every person has a nahualli. The nahualli is an animal that you have a spiritual connection to. In this form, a nahualli is much like the concept of a totem animal found in other Native American cultures. Usually, the nahualli has traits that reflect those of the person they belong to. Unlike the concept of a totem animal, the nahualli denotes an actual individual animal believed to exist, either in the spirit world or in the physical world itself (depending on regional beliefs). The nahualli is the spiritual twin of a person, said to have been born at the same time and share the portion of the soul known as the tonalli.

In the spiritual realms, a person might take the form of their nahualli at times. In other cases, a nahualli may appear much like the European concept of a familiar animal, showing itself as an individual animal of a person’s nahualli species which the person has a mental and spiritual connection to in this world. While this connection gave strength, it also gave weakness; a person could be harmed or killed by doing the same to their nahualli animal. For a person to find another's nahualli required powerful magical abilities, however.

The other definition for nahualli is a sorcerer. Nahualli sorcerers have mastered control over their animal spiritual twin, as well as other forms of magic. The most famous trait attributed to the Nahualli of legend is the ability to shape shift into their animal forms. Belief in this form of sorcery outlasted even the conquest; the Spanish friars were frightened of the Aztec sorcerers and often recorded events attributed to them. In modern Mexico, the term nahualli has been turned into “nagual,” and the concept still survives in some form.

In addition to the powers of animals, Aztec sorcerers were well versed in herbs and magical concoctions, and often seemed to walk a fine line between curer one moment and hired hexer the next. There is significant evidence that a great deal of Aztec sorcery relied on a knowledge of very real forms of seemingly mystical assassination. In the purely mystical realm, however, Aztec sorcerers were also feared and revered for their strong abilities in navigating the spiritual worlds through their dreams. A truly accomplished nahualli could strike at his enemies’ souls or heal his clients’ spirits during sleep. Such sleeps were usually preceded by extremely lengthy rituals and supplications made to the gods upon whose territory the nahualli wished to proceed. It is important to note, however, that asking permission to proceed with your will is a very different matter than calling upon the gods to run errands for you, as so many modern magical practitioners seem to believe is proper.