The ring never truly has a leader because as one power rises in the cult another eliminates them to take their place. The only leader they would listen to is the dragon of death himself. Otherwise every individual is a member and they work democratically by voting for the next decision of their cult.
Immortality can only occur through death. It's foolish to think life can be infinity, but with the right magic one can immortalize themselves in the natural world. This can only happen if life is swept aside and death arise.
The cult first made there appearance to the world and the Imperium when they killed the phoenix of life. However, they have been around for a lot longer from upwards of when the Imperium was founded to a century before. Little has been recorded about them as they are a very secretive group that has tampered with reports about them. Even the archives has not been immune to their tampering and as such, we lost a lot of reports and accounts about their activities. Whats left does peak some curiosity in their history. All sources lead to an original group that referred to themselves as the brothers. They were elves that were once powerful lords in the elven kingdoms that fought for power among themselves. Neither one could topple the other, but it was Barcinturattan who brought them together and offered them great power if they banded together to defeat his counterpart. They agreed, but each others greed didn't stop there. It wasn't long before they back stabbed each other for power and thus created the circle. A cycle of death to create the strongest person to fight for their power. Articles suggest that this cycle has lasted since their founding, but the myth seems to be a more recent addition to their history. While they haven't made another appearance in a few hundred years, this doesn't mean they are not preparing for something.