The Cult of the Nine Cobras is a dangerous mercenary-like association that appeared in the The Cobra King of Kathmandu.
In the third book, two strange robbers broke into the twins' apartment after they arrived home from Iraq. They appeared to be after the box holding the twins' wisdom teeth. Mrs. Gaunt changed the robbers into bottles of wine. Philippa finds a clue in the carpet; a flat stone with an orange snake engraved on it, which she gives to her mother. However, Layla doesn't know what the medallion is either, so she sends it to Nimrod through djinnternal mail (she swallows it; Nimrod coughs it up). It is later revealed that they were members of the Cobra Cult.
The Cobra Cult started looking up in children's medical records in order to attempt to find those who had their wisdom teeth pulled out prematurely. This allowed them to track down the djinn and steal their wisdom teeth. It would ultimately give them control over the djinn.
When Nimrod retrieved the djinnternal mail from his sister about the attempted break-in, he calls upon Mr. Rakshasas, his ancient djinn friend, who shuddered when he saw the stone. Mr. Rakshasas believed it signified that a new cult was on the loose, a Cobra cult so evil and dangerous that Rakshasas instantly feared for the twins.
Despite the dangers they could encounter with the cult, the twins and Groanin went after them with their friend Dybbuk Sachertorte, who decided to come along because his friend was murdered and his father was missing.