As a dark urge you should get the quest to kill her pretty early in act 2. I don't remember when exactly you get that quest, maybe you can skip getting it if you don't long rest.
Essential story development should not be restricted to long rests. At least not when there is a strong reason not to use long rest: powerful potions and elixirs that last until the next long rest. And
the stat boosts you can get from the secret shrine of Shar are especially nice to keep for a long time
.
