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)
You were killed. However, your body returns to life with two resident souls yours and that of an ancient, alien being. The other spirit awakened for a purpose, one that it keeps secret from your mind. There are occasional fights for control of your body.
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".