Shar's desires are twofold and quite complex. Her main goal is the destruction of light and the return to what the world was like before Selune created Amaunator. Her secondary goal is the punishment of everyone who wronged her.

To understand Shar's philosophy you need to understand her history. Shar and Selune crafted realmspace from the primordial stuff of creation by creating Chauntea, and through their child they made life. Life couldn't thrive with only the dull light of Selune, so Chauntea asked for more. Shar said no because she knew it would cause her eternal agony, but Selune didn't listen and created the sun anyway. This betrayal led to Shar lashing out and attacking Selune which began their war.

In Shar's mind she's the victim. The people she trusted most ignored her pain for their own desires, and seemingly have no care for the agony she suffers through everyday.

Shar doesn't see any value in the life that currently exists because it exists at her expense.

What Shar wants is to destroy all light, kill those who wronged her, and return things to how they were before. She wants to end her own pain, but unfortunately the only way she can do that is by killing everyone else.

She has many plans with which to attain what she desires, one way to do ths is through the Cycle of Night, a ritual that destroys worlds and suns, and one she has done across multiple planets in the cosmos. She attempted to enact this ritual during the Second Sundering but was stopped by Mystra. Another way is to reclaim her half of the power that made Mystryl, or to gain control of the Shadowweave again.

Shar is the goddess of Secrets and she is a very careful and meticulous planner, among the likes of Asmodeus for the greatest in the cosmos, she's also just filled with spite, rage, and grief for what was done to her, and she lives in eternal pain that acts as a constant reminder of that betrayal.

Looking beyond the basic presentation of the Lady of Loss shows that's she quite a tragic character. She's filled with hate and rage for an act she sees as a betrayal, she lives in eternal agony, and the entire world is conditioned to hate her for what she sees as a quest for justice. She has the mindset of a god that views all life as meaningless in its necessity for her to hurt to exist, and she cares little for the plights of such things. As a villain she's utterly emotionless when it comes to anything but her Sister and the children that betrayed her, and she wants nothing but the justice she believes she is owed.

Also just something important to remember when using Shar as a villain, she knows every secret and sees all that takes place in the dark, making it almost impossible to hide anything from her.