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)
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".
Monster. A monsterous enemy could be something as weak as a goblin to as powerful as a ancient dragon. They consist of monsterous races and creatures. They might have done battle with the creature, been robbed by it, wanting revenge, or hunting it.