Secret Orders are almost as old as clans themselves. Secret Orders often function as secret societies in Clans involved with roleplaying
There are two main types of Secret Order
Sub-Clans are the most common type of Secret Order because they are the easiest to organize. They function within a larger clan though act in some way seperatly. Many are Elite members of thier Clan, while others serve another purpose, such as Hazeel Worshippers within a Clan that Roleplays as Zamorak worshippers, while others are the clans Assassin's or Specail Ops Force.
Covens are harder to organize but often play bigger roles in thier roleplay. Covens are Secret Orders whos members are a small portion of more then one clan. They are often Illuminatti Styled organizations that try to influence a specific outcome within thier collection of clans or even a whole Alliance or Union.