"The all live happily ever after...but they don't."
The Heroes, by whatever means, defeat the BBEG. They mourn for and bury their fallen companions, and then try to return to their previous lives.
Except, they can't. One has constant, reoccurring nightmares about the time she was captured and tortured by the BBEG's minions. Another is left a shellshocked wreck, who jumps at the slightest loud noise. The Paladin, having seen such depths of evil, slowly begins to see evil in the hearts of all men and falls after slaughtering a village for 'deviant thoughts'. Without his powers, he is lynched by the survivors.
Marriages and relationships with surviving family members break down. The heroes complete their goal, but it leaves them all burned-out, miserable wrecks unable to function in society.
The lucky ones died a heroic, climactic death, and were spared such a terrible, undignified fate.