Final answer:
The character that fits this description is Goodman Brown from the story 'Young Goodman Brown' by Nathaniel Hawthorne.
Step-by-step explanation:
The character that was originally based in part on a Celtic bard who went mad and lived in the woods as a wild man after his tribe was nearly wiped out in a decisive battle is the character of Goodman Brown from the story 'Young Goodman Brown' by Nathaniel Hawthorne.