It all started with a letter and a ring
The appearance of a little girl no more than 13, with a pock mark scarred face from a disease in her past. She was the sole survivor of a plague that destroyed her village. Before the plague broke out, a man rode through town and talked with several people in town then left. The girl goes out in search of this man to find out the cause of the plague. She follows him through cities dying of the plague, until eventually she catches him. When the man completely ignores her and her questions, her mind breaks with anger and grief and she stabs the man to death with a small hunting knife.
Now, wherever she goes, the disease that ruined her life follows her and devastates anyone she meets. She is driven by loneliness and curiosity about the peoples of the world she should never meet.
Occasionally she has outbursts of rage and blames the world for her affliction and causes the destruction of a town or country.