Very common? How common can an underground doomsday cult be?
Um...VERY! Shar is the goddess of darkness and the caverns of Faerun as well as a neutral evil GREATER deity of Faerun. She is the twin of Selune. She was the creator of the Shadow Weave which is the counterpart and attack on the Weave. According to one of the most ancient myths of the creation of the world and the heavens, after the universe and its crystal sphere were created by Ao, Shar coalesced from the primordial essence, alongside her twin Selune. They were considered 2 of the oldest gods in Faerun and they've been at war for countless millenia.
So yes, I would think everyone would know Shar and her symbols about as much as everyone in our world would know about Satan and his demons. Even in more recent history in Faerun, Shar and Selune battled in the streets of Waterdeep, so I would think that at least History and/or Religion rolls should be made by all parties involved the moment anyone would see Shadowheart in her "I'm a Sharite Crusader" gear, and the difficulty should probably be no more than 10.
