A vicious animal attacks you on a moonlit night. Friends drive the beast off, but you discover that you've been cursed with lycanthropy, to become a similar beast under the full moon. (work with your DM to determine details)
Martial training: In adolescence you trained with the weapons of your people and learnt how to use them proficiently (work with your DM to determine details)
In adult life you gain a rival who seeks to undo your character, they will attempt to hinder your efforts until they get what they want. (work with your DM to determine details)