It's been two centuries since the war.
Lazuli wasn't allowed the freedom of death. While everypony she knew and loved was killed or simply died of old age, she remained alive. Her magically immune body made the radiation a non-issue, but life was hell for many years.

Now, things have cleared up. She works as a contract killer and mercenary in the Wasteland - but demons from her recent past and angels from before the world ended will make her somewhat simple life a lot more confusing.