You were abducted by otherworldly beings (planar, fey, fiend or celestial) Moments later, you reappear, but has physically aged 1d10+10 years, has no memories of that lost time, and has not mentally aged even a minute.
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)