An iara is a vampiric spirit or vampiric witch from Brazil, depending on the way it died. If a person dies violently, or before his time, or outside the Catholic Church, or if a body is not given a proper Catholic burial or is buried in the jungle, that person will become the vampiric spirit type of iara. However, if a living person sells his soul to the devil for power, he will become the vampiric witch kind of iara.
The iara, no matter how it came to be, can, in its human guise, sing a beautiful, sirenlike song that will lure men out into the jungle. There is a protective chant that can be uttered as soon as a man hears the iara’s song, but he must be quick, otherwise he is doomed to fall prey to it. Once the iara has secured a victim, it shape-shifts into a snake with red eyes and, using a form of mesmerism, hypnotizes its prey, after which it will drain off his blood and semen. It leaves the bodies of those it has killed near waterways.
Source: Bryant, Handbook of Death