You survive a deadly encounter with a non-humanoid monster. Because of the encounter, you gained insight into the creatures weaknesses. (work with your DM to determine details)
You are mistaken for the missing offspring of the ruler of the land and are taken to live in the ruler's household. After 1d6 years something happens that causes you to leave. (work with your DM to determine details).
As an adult you joined a rebellion against the local ruler, if successful you are outlawed, if it failed you escape, with only friends and family knowing your involvement. (Work with your DM to detirmine the details)