Shar (Forgotten Realms) - Dark rituals that give powerz to PCs (Pls help)

One of the players is a Cleric of Shar (level 13 now), and he has kept his deity a secret from the other players since the beginning. However, due to recent events, it's becoming clear that his secret will soon be exposed.

The player has spoken to me about possibly revealing his true nature to the rest of the party, but he wants to see if they will join him in his worship of Shar. He would like for me to create a dark ritual or something that would entice the other PCs into joining him. He'd really like it to be something that will advance in power as they level up, so it's not a short-lived allegiance.

Since Shar is all about using hidden cult cells to do her bidding, this is perfect. The Cleric of Shar could easily be seen as the "cell leader" and the other PCs, even without levels in cleric, could be seen as cultists or followers.

I want to offer something that the players will like, and I really like the idea of possibly enticing the other PCs to begin worshiping this dark god. I want the dark ritual that they enter into to be powerful enough to be really attractive from a player perspective, but not overpowerful. I also want the ritual/powers to be in the vein of Shar. It should be about shadow, darkness, and evil. And hate for Selune, the moon goddess and Shar's twin sister.

Ideas I've had are:

* A free level in cleric (not sure about this, too weak? too strong?)

Maybe the ritual offers a few different things to choose from and each PC gets to choose only one (whichever they want). Each time the ritual is performed (which can only be done once every two levels of the cell leader), the members can choose another power. These would be things like:

* +1 Profane bonus to AC
* +1 Profane bonus to Attack
* +1 Profane bonus to All Saves
* +4 Profane bonus to Gather Information, Sense Motive, Bluff, Hide, or Move Silently (pick one)
* Spell-like ability usable once per day (but what ability? something shadow-oriented no doubt -- maybe a spell from Shar's domain spells? And what level of spell would be allowed? Maybe, it could be anything from Shar's domain spell list that the cleric cell leader can already cast?)

Drawbacks: I want there to be some secret negative to performing the ritual too. I'm not sure I want to take the heavy-handed approach of automatically changing their alignments to evil, but this is definitely an evil act.

I had the idea of using Gaes/Quest. Basically, the cell members would be compelled to obey the cleric cell leader in matters of the church as though under the effects of Gaes/Quest. The cell leader could not use this power to force a member to do something non-church related, like "Fetch my shoes," but he could use it for matters of faith, like "Kill that Selune follower."

As for the leader of the cult, his "negative" is that he now has a personal relationship with Shar. She will communicate with him through his dreams and in the dark of night to make her wishes known to him. She will expect him to obey her desires and carry out her commands. Failure to comply will be met with some kind of punishment (loss of WIS maybe? or loss of spells?).

I'm looking for any suggestions or feedback on this idea.

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