- "The-there's a reference in here to a Brotherhood of seven demons who take, uh, the form of young humans. Every seven years these demons need human organs, a-a-a brain and a heart, to maintain their humanity."
- ―Rupert Giles[src]
The Brotherhood of Seven was a group of demons who took the form of young humans. Every seven years they sought out human brains and hearts in order to maintain their human form, otherwise they would revert to their true visages. Sometime in or after the 1930s, they came into conflict with demon hunter Sid. Rather than kill him, the Brotherhood cursed Sid to have his soul trapped in a dummy forever, a curse that could only be broken with all of their deaths. Over the next 60 years, Sid hunted down the Brotherhood, killing six and tracking the last, Marc, to Sunnydale. Sid teamed up with student Morgan Shay in an attempt to find the last demon and came to believe it was Buffy Summers due to her unnatural strength. After Sid and Buffy learned the truth about each other, Sid teamed with Buffy and the Scooby Gang to finish Marc. After discovering Marc was targeting Rupert Giles, the gang rushed to his rescue in time to stop Marc from getting the last brain and battle him. The last of the Brotherhood was finally defeated by the Slayer and Sid and decapitated with his own guillotine by Xander Harris. Knowing that wasn't enough to finish him, Sid stabbed Marc in the heart, killing him. With the last of the Brotherhood of Seven dead, Sid's curse broke and he died too as his body was long gone.
CharacteristicsTo maintain their human appearance every seven years, these demons needed a healthy heart like Emily or a brain healthy or perfectly functional as that of Giles. Buffy found that Morgan's brain, which had a tumor, was discarded as it was inedible for Marc. It was not specified if the taken organs needed to be ingested, transplanted.
The brotherhood were spellcasters and successfully performed at least one to curse their enemy, the demon hunter Sid. They also possessed superhuman strength in their demon form as Marc was able to smash his way out of a coffin while, as a human, he needed to use a knife to collect the organs while his demonic claws would've been sufficient enough. Also, based on Marc' point of view, these demons appeared to view the world in a purplish hue.
Unlike most demons who can be killed by decapitation, the Brotherhood can survive it, though it incapacitates them for a time. Presumably they eventually regrow or reattach their heads as the only way to truly kill one is to destroy their heart.