Really, I think you would need to make some kind of Application for this if you want to play it, but I'll post it anyways.
The Cult of The Dragon is a cult of humanoids that wish to serve undead dragons and animate them as well, acting as information systems, and providing the dragons with a horde. They truly believe that dead dragons shall rule the world, and they hope to gain something from this by serving them before anyone else devoutly such as being kept as monarchs in line. They are a semi-religious evil cult, usually loyal to Tiamat. The Cult of The Dragon extends into underground regions; this is a reason they would be down in the Underdark. Though there was a little misunderstanding.. Simply "dragons" will rule the world became "dead dragons". Though the cult's intentions we're still obvious: serve dragons, as they will supposedly rule the world.
WHAT COULD BE IN THE CULT
Alignments: Any evil, neutral is possible. Gods: Numerous ones are possible; think for a while and you'll come up with some, such as Velsharoon, Tiamat, Bane (It's a Banite priest that's currently in control. And that's what the cult is mostly consumed with.) and numerous more. Classes: Any but Druid and Paladin; infact, these two particular classes are completely against the cult.
WHAT YOU NEED TO KNOW
The founder was Sammaster, a lich, also one of Mystra's Chosen. He is currently said to be dead in his phylacartery somewhere.
Lathander is strongly against the Cult, as well as gods of death such as Kelemevor, Urogalan, Jergal, etc.
The current leader is Algashon, a priest of Bane.
Sammaster convinced dragons to become dracoliches by switching around some words on the transcription.
The Cult of The Dragon may ally with drow on occasions, due to their location.
Senior cult members are known as Keepers of The Secret Hoard, or more recently, Wearers of Purple. They are given the job of Dracolich creation.
THIS EVIL AND INSANE CULT NEEDS YOU.
(Someone please tell me if this needs an application to play.)