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)
Rivalry: A rival to you or your family cause upheaval forcing you to relocate possibly to a far distant land. (Work with your DM to detirmine the details)
When you become an adult you learn to used a weapon (of your choice) appropriate for your culture and social standing. (work with your DM to determine details)