As others have said, this story (The Eye of the Beholder) is to Two-Face as The Killing Joke is to the Joker. It was the first story to make Dent's initial scarring the last straw in a lifetime of uphill struggle to stay sane, as opposed to the original Golden Age origin where the scarring abruptly (and implausibly) turned a well-adjusted and happy D.A. into a deranged villain. And therefore it was the first story to make Dent a genuinely tragic figure. (Although the Two-Face story in the first chapter of DKR arguably also qualifies, if for a different reason: the surgery intended to bury Dent's evil side has the unforeseen result of destroying his good side.)