Shar

"This is the battle you and I must fight forever, Shar—night devouring the light of the moon, the moon replenishing itself. That is our place in the balance of nature. The fragile, eternal balance Lord Ao has charged us gods to uphold."

- Selûne to Shar during their battle over Waterdeep

"In the twilight of battle, one sister falters, But Selûne hurls magic in desperate power. From two sisters, one child the balance alters— Mystryl's aid to Selûne ends Shar's dark hour!"

- An excerpt from a ballad by Veseene the Lark

Shar, also known as the Mistress of the Night, the Dark Lady and the Lady of Loss, was the goddess of darkness and the caverns of Faerûn, as well as one of the main antagonists in Forgotten Realms franchise, a part of Dungeon and Dragons universe. Shar despises the life-bearing universe Selûne created and sought to wipe it out.

Origin
Shar is one of the oldest and most evil of all the gods in Dungeons of Dragons franchise. Being the evil counterpart to her twin Selûne, Shar is the ruler of Shadowfell, presiding over caverns, darkness, dungeons, forgetfulness, loss, night, secrets, and the Underdark.

Shar was also the creator of the Shadow Weave, which was a counterpart and attack upon the Weave, controlled by Mystryl and her successors, before both of the Weaves fell into ruin during the Spellplague.

After being defeated by her sister through their nigh-eternal struggle, Shar formed the Church of Shar based around loss and nihilism with her clergy subverting and destroying hope itself.

Spellplague
Taking a more proactive role in the modern day, Shar helped to destroy the goddess Mystra and unleash the Spellplague, later having her chief worshippers unleash the Shadow Storm to blanket all Faerun in life-stealing darkness.

Shar's ultimate goal is to start her prelude to the Cycle of Night, a event she would bent on extinguishing all lifeforms and rule the empty universe full of nothing but void and darkness, with pure enjoyment and estacy.

Trivia

 * Those who believed in the Dark Moon heresy held that Selûne and Shar were two faces of the same goddess.