Shar, the Mistress of the Night, was the goddess of darkness and the caverns of Faerûn, as well as a neutral evil greater deity.
I assume became a major deity around/after the Time of Troubles (1358 DR) and when she "absorbed most of the divinity of her son Mask (a god)" in 1374 DR, created the Shadow Weave (sometime), helped kill Mystra to cause the Spellplague and created an entire plane of existence - Shadowfell - in/around 1385 DR. Those were some majorly impactful events and growth of personal power. (BGII took place in 1369 DR.)
