Demons are very powerful maleveolent spirits with supernatural powers. Although the existence of demons have yet to be explained, based off of the HBO original series True Blood and its predecessor The Southern Vampire Mysteries, demons are powerful beings that possess superhuman speed and strength comparable to vampires. Demons are also known to have control over fire, and are telepathic. While these characteristics did not resume in the series, Jesus' family are the first witches ever known to possess demons within their bloodline.
Searching for help to protect themselves against Vampires, 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 succeded, 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.