The Twilight Sisterhood

The Matron → The High Priestess → Covenmother → Priestesses

Myths abound regarding this secretive faith. And no one but those within the Sisterhood know the truth of them. And even then, they are given more knowledge as they rise through the rites (ranks) of the faith. Some popular myths include that only Shadar-kai are chosen to fill the role of High Priestess. Or that the Raven Queen reveals her name to the High Priestess who then keeps it secret. That the priestesses of the Sisterhood give up their identities in order to raise through the rites.
Any number of these myths, and any elements of them, may be true or false. And perhaps the Raven Queen likes it this way. As the confusion over her and her faith serves to maintain power and authority over those who do not know the truth of these things. "Knowledge is Power", after all.

The Sisterhood embodies the desires of the teachings of the Goddess of Secrets. The first lesson taught by the Raven Queen is that one unspoken: The power of names; as the Matron of Night keeps her's a secret to assert and retain power over others.

The Twilight Sisterhood is a female only priesthood. Each priestess through expressed and experienced devotion to the Raven Queen and the Sisterhood are inducted into the various rites of the faith, or the ranks of the priesthood. The rites are:

First - The Rite of the Hidden Name
Second - The Rite of the Smock
Third - The Rite of Silver
Fourth - The Rite of Silence
Fifth - The Rite of the Mask

Each of the rites serve as a lesson and instruction for each priestess of retaining their identites for themselves, their sisters, and the Raven Queen as well as to the seriousness of their charge as keepers of knowledge and preservers of the sanctity of death. Each rite has an attending ceremony of induction.

The Rite of the Hidden Name and its induction ceremony are also the induction into the Sisterhood. They covenant to the Raven Queen to keep their name a secret from anyone outside the Sisterhood; to quote the Raven Queen: "You shall retain your name unto me, and share it only in the presence of your Sisters." The induction ceremony of this rite (and to the Sisterhood in general) is an anonymous vote to either allow or bar the entry of the proposed sister. The Covenmother will assign one sister already inducted to vote no; this mystery voter remaining a secret, establishing in the unsuspecting inductee's mind the power secrets hold over those without that knowledge.
The Rite of the Smock (sometimes called the cassock) is the first clothing ceremony of the Sisterhood. Inductees to this rite are given a coarse, roughspun cassock meant to hide and obscure the form of the body of the sister. The roughspun nature of the cassock makes it initially uncomfortable to wear, serving to challenge the devotion of the priestess. They are also given a habit to begin to hide and cover their head. Inducted sisters are also instructed to not style their hair but only let it lay as it naturally would; only cutting their hair when it interferes with the wearing of the habit or the smock.
The Rite of Silver is the first rite to be considered in the "upper echelons" of the faith. Only about half of the priestesses inducted into Sisterhood will ever make it to the Rite of Silver. In this rite the priestess is given a charge to destroy undead or any other mockery of the cycle of life and death. The rite is composed of and completed by sending a priestess out on assignment to destroy or disrupt necromantic affairs. To achieve this, the sister is given a silvered weapon, hence the name of this rite. When (or rather, if) the sister returns from their assignment, they are inducted into the Rite of Silver.
With the Rite of Silence, a priestess of the Rite of Silver must be recommended by a sister of the Rite of Silence or the Rite of the Mask to the Covenmother for advancement. If accepted, the sister is sworn into a vow of silence, the duration of which being determined by the Covenmother. The induction ceremony of this rite is the most controversial for those who are aware of it. The inductee is sat between a group of sisters from the Rite of Silence or the Rite of the Mask who take turns smiting or whipping the inductee with rods, whips, or switches; supervised by the Covenmother. If the inductee makes any noise during this ceremony, the ceremony is adjourned and the inductee must request a resubmission to the ceremony. This repeats until the sister successfully takes her stripes to the satisfaction and judgement of the Covenmother. A notable aspect of the rite for those successully inducted is they are given much knowledge considered too sacred to share with outsiders or sisters of the lower rites, the newly inducted sister only able to silently accept these teachings, unable to question.
The Rite of the Mask, the highest rite within the Sisterhood, follows a similar process as the Rite of Silence, except that the Covenmother must recommend a petitioning sister of the Rite of Silence to the High Priestess for approval. It is up to the Covenmother to determine when a sister has observed their vow of silence long enough to petition for advancement. The High Priestess then meets with the sister and tests her allegiance to the Rite of Silence, saying that the petitioning sister is free to speak in the presence of the High Priestess. If the petitioning sister endures the test, the High Priestess inducts the sister into the Rite of the Mask by gifting her a porcelain mask, similar to that which the Raven Queen is oft depicted as wearing. The Rite of the Mask then serves as a retroactive freeing from certain aspects of previous rites. Such as the coarse smock recieved in the Rite of the Smock is replaced with a gentle, silk cassock. And the sister is now relieved of her vow of silence and can speak freely. Sisters of the Rite of the Mask are the first and foremost spiritual leaders and guides within a coven. The sister, like with the habit from the Rite of the Smock, are encouraged to paint, embroider, or otherwise decorate the inside of their mask, speaking once again to the importance of retaining your identity unto yourself and the Raven Queen. It is then only in coven sanctuaries that sisters are supposed to ever take off their masks.

Each Covenmother, the leader of a coven of the Sisterhood, is chosen personally by the High Priestess from those inducted into the Rite of the Mask. They are officially the spiritual leader of the coven, but usually leave most of the teaching and instruction to the sisters of the Rite of the Mask; leaving the Covenmother to be much more of an administrative position, though still concerned with maintaining orthodoxy in her coven.

For the regular priestesses, even those who have risen through all five rites, are not granted any particular powers or abilities beyond that of regular cleric and paladin abilities/magic. However, as sisters rise through the rites, unique effects begin to originate from them. Some experience a light aura of death where things like plants begin to die or prematurely age when a priestess hangs around them too long; sometimes doors or windows will begin to creak open when a priestess is around; some will hear random extra footsteps when walking with a priestess; flowers, particularly roses, will sometimes seem to bend in the direction of an approaching sister, etc.

The High Priestess, however, is said to be granted the Raven Queen's ability of knowing and/or seeing when any given person will die. This information of course the High Priestess is sworn to never share with anyone, as members of the Twilight Sisterhood strive to not influence the time of a person's death.

When creating a character that is a member of the Twilight Sisterhood, consider some of these mechanical changes; and note that any restrictions you choose to have do not have to be run as a hard and fast rule, your character can have as much agency on these matters as you would like them to have.
Members of the Sisterhood are generally forbidden from casting necromancy spells, specifically revivication spells. To use general necromancy spells is more just frowned upon, but to use revivication spells would be considered anethema to the Raven Queen and the faith. However, being a faith dedicated to preserving the sanctity of death and of secrets, perhaps the most anathema spell a member of the Twilight Sisterhood could cast would be the Speak with Dead spell; where you violate and reanimate a corpse to pry from it its knowledge it had.
With the Twilight Sisterhood being dedicated to secrecy (as experienced by practices like the Rite of Silence) you may consider limiting yourself to spells that do not require verbal components.

Different sects of the Twilight Sisterhood would be considered by the Raven Queen a flaw, not a feature. Most sects and schisms within the faith form along various liberal and conservative axes of interpreting scripture and the rites. On many issues the Raven Queen will have the High Priestess instruct the various Covenmothers and their covens on corrections to different interpretations of the words of the Raven Queen and the rites.
Some examples regarding the rites include: Different covens interpret the charge over the Rite of the Hidden Name, "You shall retain your name unto me, and share it only in the presence of your Sisters." differently (again, to the frustration of the Raven Queen) from the most conservative adherents saying you should only speak your name within guarded areas of Sisterhood sanctuaries, (as you never know who may be around) to the more liberal interpretations who say that you can say your name in any situation so long as a member of the Sisterhood is also present.
Another example comes from different interpretations/enforcements regarding the Rite of Silence; the more liberal branches of the faith have this referring only to speech; while the conservative persuasions of the faith extend this to referring to any sound at all, including that as a result from pain (which is not without precedent considering the induction ceremony of this rite).