The Brotherhood is a clan with all it's clan members taken an oath of allegiance to the Moonstone Opal. All are apart of the Dark Kingdom and whoever wields the Moonstone can use the Mind-Trap Stone to command the group.
When the Moonstone Opal created the Dark Kingdom, passing generation kings had been aware of it's great power. When Edmund became king, he made sure to keep the power a secret so any looking to do no good could not possess it's power as it could mean doom.
Edmund decided to send out his most trusted knights to keep the Moonstone Opal a secret from others, they then named themselves the Brotherhood.