Betrayal: You were personally betrayed by someone very close and dear to you causing your great heartache. (Work with your DM to detirmine the 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 adulthood you have an encounter with an unusual person, who as far as you can tell, has no home or family. You offer to help in anyway you can. After accepting your hospitality the stranger vanishes leaving a note "Your kindness will be remembered".
Faction. A factional enemy could be a religous order, a merchants guild, an order of assasins, or an order of holy knights. It is a enemy that may have agents and spies taht act of their behalf. Could be on the run, part of a rival faction, or want to join