A deity or other such powerful being asks you to become its agent on earth, if you choose to accept The deity gives the character an unusual pet as a gift. (work with your DM to determine details)
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.