Its not a person, a mortal, thats for sure. The way whole thing is talked about, the actual spells priesthood gains, the fear tadpole causes to even someone like ancient hags...
Its a god-ess. If BG games have a common thread its the "Gods play their games with mortals" theme. One of the dead three played the main villain role in the originals, the signs of the whole dead three are all over the start of the game - but they are not enough anymore, because they have been reduced and simply repeating the story of the originals all over again would be dumb and cheap. One of the most basic things sequels try to do is to top the previous movies or games or stories. The presence and influence of Shar is all over the story. From companions to whole sections of game world. And she already used the dead three as her pawns, killed and impersonated other gods, including giving their spells and powers to the priests to maintain the charade.
