A Demon is a very dark and powerful spirit that possesses supernatural powers. On the HBO original series True Blood, the basis of what a demon is remains unknown, however, it is possible that much like faeries, demons are a race of supernatural beings that are native to an extraterrestrial dimension. They possess superhuman speed and strength that is comparable, but inferior, to vampires, and are known to have control over fire. Demons are also believed to be telepathic, and are able to identify the thoughts and desires of others. Some witches, especially brujos, have been known to summon demons from their realm as a means to enhance their power. While demons appear to be very servile to their master, they are also very violent and cruel by nature. A brujo by the name of Jesus Velasquez has reluctantly summoned demons for help in extreme cases of emergency, and his bloodline is the first to feature demons throughout.
Searching for help to protect themselves against vampire Eric Northman, Jesus and Lafayette travel to Mexico to meet Jesus' grandfather, Don Bartolo, a powerful brujo adept in the dark arts with a demon inside him as well as Jesus. He summons his demon side while making a snake bite Jesus' neck. ("I Wish I Was the Moon")
Without Antonia, Marnie, now a ghost herself, possesses Lafayette in order to force Jesus to give up his demon to her. Marnie succeeded, and in the process killed Jesus. Marnie uses the demonic powers to attack and nearly kill Bill Compton and Eric Northman, but is stopped by Antonia herself (summoned by Holly Cleary, Sookie Stackhouse and Tara Thornton). Though Marnie crossed over, the demon remains with Lafayette. ("And When I Die")
Jesus' demon is still in Lafayette and is being drawn out when Lafayette experiences anger, causing him at one point to curse Sookie's car and nearly kill her. Lafayette then attempts to summon Jesus, seeking help in expelling the demon from him, only to see Jesus head appear with his mouth sewn shut. Ruby Jean tells Lafayette that Jesus is in danger with Don Bartolo. While at the sorcerer's house, Bartolo attempts to steal Jesus' demon out of Lafayette, but Don's wife kills him in the process.
Due to the fact that hell is nonexistent in the True Blood universe, it is most likely that demons are native to an extraterrestrial dimension, as oppose to the biblical underworld that most people believe. As spiritual beings, demons are truly immortal and cannot die or be destroyed. However, if a demon were to be defeated, it is most likely that the demon's essence would simply return to its original realm.