About Shar and her followers:
A Sharran is a different breed of evil than you generally find here on the server. They do not seek to conquer. They do not seek to rule. They do not kill for the pleasure of killing. No, the evil of Shar is a subtle thing, and far more sinister than most are accustomed too. It is a poison that eats away at their souls and it is a disease that they seek to actively spread to others.
They spread their darkness in many ways, but most often those who find themselves under a Sharrans sway do not even fully realize it until it is too late. Loss, bitterness, hatred and vengeance are all seeds sown by a follower of Shar, and Sanctuary is a place ripe with these emotions. Even the most noble of Paladins are not immune to Shar's dark touch.
A Sharran is vile. They seek to ruin lives, relationships, businesses, all in an effort to spread loss. It is believed that through these actions the individuals involved will ultimately find themselves moving closer to Shar. This is not necessarily due to the loss itself, but more often due to the negative reactions and emotions associated with the loss.
However, this is not the only type of Sharran you can or will find. At the lowest level some who join in worship of Shar simply want to belong to something greater than themselves, something dark and naughty. They may not even really believe that Shar can see, hear or touch them in some way, until of course, they are called into her service, and then it is often too late to turn back. Others, like the above mentioned are the truly committed, those who are deeply dedicated to bringing about the dark goddess ultimate goal of nihilism. It is the effective equivalent of mass murder and suicide all at once, the desire to see everything that exists and ever will exist destroyed including themselves.
Others may find themselves drawn to Shar for the belief that she can ease the suffering and pain in their lives, and this is especially true in the Underdark where there is an unending night. Some may feel embracing Shar is necessary to survive in the Underdark, while others embrace her to further their own goals effectively selling their souls to an ultimate evil to further their own (often immoral) objectives.
No matter the reason people find themselves drawn to Shar they are all united and bound by secrets. This ensures that if anyone betrays another, which is rare because they are united in faith they will end up destroying themselves in the process. No one can truly join a Sharran cell without committing some dark deed that can be used against them should they betray their faith. (Not to mention a very, very bad afterlife.)
All of the above describes, in the briefest and most general of terms what type of individuals find themselves in the service to Shar. Virtually speaking, you can exist in any other faction within Sanctuary simultaneously. A Sharran can easily blend in and go unnoticed and this is often their desire. Most Sharrans do not wish to lead openly, but rather work and manipulate from the darkness.
Why, Shar? Why?