Mictlantecuhtli is ruler over Mictlan, the lowest underworld, the northern realm of the dead. Mictlantecuhtli's wife is Mictlancihuatl. He is associated with the spider, the owl and the bat.
To create life, Quetzalcoatl needed the bones from those that lived in the previous world, the Fourth Sun. First Mictlantecuhtli agreed but when Quetzalcoatl came to collect the bones Mictlantecuhtli changed his mind. Fortunately, Quetzalcoatl managed to escape. However, on his way back up he dropped some bones and broke some of them. This explains why humans have all different sizes.
Mictlantecuhtli is both the ruler of the tenth day, Itzcuintli (dog), and the tenth trecena, 1-Tecpatl (knife). He is Lord of the Day for days with number 6 ("chicuacen" in Nahuatl). He is the fifth Lord of the Night.