The Aztec god of rain.  One of the two principal Aztec deities, Huitzilipochtli being the other, Tlaloc's priests commanded one of the two principal temples in the center of Tenochtitlán. He is always depicted with big round eyes and fangs.

Now, Brittanica Online (at www.brittanica.com) has an extensive article about Tlaloc.  It will tell you his name is Nahuatl for "He who makes things sprout".  It will also tell you his consort is Chalchiuhtlicue, “She Who Wears a Jade Skirt”, goddess of fresh water rivers and ponds.  While Tlaloc sent rain to the people, he could also send drought, lightning, and hurricanes to manifest his anger.  The article will also tell you that in order to appease Tlaloc, the Aztecs would sacrifice children to him two times a year.

Brittanica does not mention that one of the most important (perhaps the most important) events in Mexican history involved Tlaloc (which I learned watching The History Channel).

Shortly after Hernán Cortes entered the Aztec empire in 1519 and was received by the emperor Moctezuma as the second coming of Quetzalcoatl, Cortes made his host Moctezuma a prisoner within his palace at Tenochtitlán when the people of that city began to grow angry, suspecting that the visitors were not returning gods after all (He and his men were more interested in gold than diplomacy).  Cortes left Mexico and traveled to Cuba for reinforcements.

While Cortes was gone, an important festival sacred to Tlaloc came around.  Instead of human sacrifice, however, this ceremony involved the priests of Tlaloc dressing in feathered costumes and dancing around the city shaking sacred rattles.  The "bird men" apparently became a focus for demonstrations against the Spaniards.

You can probably guess what happened next.  The Spanish were spooked by the Bird Men and fired upon them.  The murder of Tlaloc's priests caused the whole city to rise against the Spaniards, who were besieged in the Palace. Moctezuma offered (or was ordered) to speak to the people and calm them.  At the critical moment, someone killed Moctezuma.  This may have been a Spaniard, this may have been one of Moctezuma's subjects.  The Spaniards barely escaped with their lives; Cortes returned to find them taking refuge with the Tlaxcalan (none to happy with the Aztecs who had subjugated them).

The Bird men have begun shaking their rattles in Mexico City again. It is probably not surprising that Moctezuma's Revenge involves water.