The Volturi

The Volturi are the largest and most powerful coven of vampires. They enforce the laws of the vampire world.

The equivalent of royalty in the vampire world, the Volturi consists of 5 core members: Aro, Caius, Marcus, Aro's wife Sulpicia, and Caius's wife Athenodora. Marcus's wife (and Aro's sister) Didyme was also a member before she was killed by her brother. Aside from these head members, it also consists of a collection of powerfully imposing vampires, known as the "Volturi guard", the most powerful being Chelsea, Jane and Alec.

The Volturi act as the unofficial royalty in the world of vampires, and are an incredibly influential coven. They are also considered 'nighttime patrons of the arts' as, because of their inability to sleep, they study the arts at night. The Volturi act as guardians, keeping the secret society of vampires hidden from the human world as needed. They often send their agents to travel from Volterra to destroy overzealous covens from exposing vampires through mass eradication of every vampire (and any humans) present. The coven has existed for over 3000 years, and is the largest coven in existence, followed by the Olympic, the Denali, and the Mexican covens.

Laws:

Throughout the centuries, the Volturi have established and enforced a number of laws that all vampires are expected to obey or get sentenced to death penalty if they don't. There aren't many of them, and they mainly concern the secrecy surrounding the existence of vampires.

The laws are not written down; to write them would in itself be an infraction. Instead, the laws are passed by word of mouth from vampire to vampire.

• Hunts must be inconspicuous, with victims unlikely to be missed; their remains must be disposed of and territory must be changed often.
• Incapable of self-control, and therefore a threat of exposure, immortal children are not to be created. Such creation is the ultimate taboo and punishable by death to both child, the creator, those who defend the child and those who know of its existence.
• Dealing with Children of the Moon, except for the purpose of extermination, is prohibited.
• False witness, regardless of intent, is punishable by death.
• Hunting is forbidden in Volterra, the town of residence of the Volturi; their food is brought from outside, sometimes from quite far away.
• Attention should not be drawn and vampires should not be noticed in open sunlight.
• Creator is responsible for his newborn's behavior and teaching of the laws before they can be released on their own. 
• By letting a newborn hunt conscpicuously, the penalty will be death both to the newborn and its creator. Even if the newborn is ignorant of the law, his rash acts are still punishable by death.

Vampires' interaction with humans must not draw wide attention. If a human becomes aware of them, the vampire at fault would be responsible for silencing that human. This mostly leads to two options: either make the human join the immortals or silence that human permanently.