While Dragon Ball is often known for its over-the-top action and fight scenes, the series has never been afraid to tug at the audience's heartstrings, with many arcs showing the harsh reality of war and the inherent sadness of battle. Many different parts of Dragon Ball can be sad and heavy with emotion, but there's a relief that the most tragic story in the franchise never actually happened.
1993's TV special, Dragon Ball Z: The History of Trunks, is widely viewed as the saddest story in the series' history. It's set in an alternative timeline where Goku dies of the viral heart disease after defeating Frieza, but as this is a natural death, he can no longer be revived by the Dragon Balls. Without Goku around, Android 17 and Android 18 quickly lay waste to the Earth, killing all of Piccolo, Vegeta, Krillin, Tien, Yamcha, and Chiaotzu in quick succession. With Piccolo gone, Kami also dies, rendering the dragon balls useless.
Basically all alone and desperate to protect the Earth, the surviving Gohan agrees to train Trunks despite the boy's age. However, no matter what he does, Gohan cannot get his young friend to go Super Saiyan. When the half-Saiyans attempt to retreat from their following battle against the Androids, Gohan tragically loses his arm. Trunks continues to train but the situation doesn't improve, and Gohan is heavily weakened due with just one arm remaining.
When the diabolical 17 and 18 attack once more, Gohan sneaks off to fight without telling Trunks. The latter quickly follows, however, only to find his mentor and best friend face-down in the street, dead. Trunks' anger, rage, and sadness combine together, and he finally reaches Super Saiyan. Alas, even this isn't enough, and the world's last hope is for Trunks to use Bulma's time machine to go to the past, cure Goku's heart disease, and help turn the tide against the Androids.
The History of Trunks has never been topped when it comes to Dragon Ball sadness. While defeat and death are not uncommon in this franchise, the heroes tend to sacrifice themselves for the greater good in powerfully tragic moments. And these sacrifices are often rewarded, allowing other heroes to win the battle thanks to such noble actions. However, The History of Trunks is a slaughterhouse with many Z fighters and other heroes suffering pointless, meaningless deaths. Gohan's demise is the perfect example of this, as while it allows Trunks to finally go Super Saiyan, it isn't enough, meaning he gave his life for nothing.
With an oppressive atmosphere that no other piece of Dragon Ball media has come close to, The History of Trunks is one of the only times where the odds appear totally unbeatable. Without Goku, it truly feels like the Androids have won. Even Trunks' final action of going back in time feels less like a clever heroic gambit and more one of resignation, the act of a man who realizes all is lost and nothing can save that timeline.
Thankfully, the ending of The History of Trunks offers some hope by undoing the events of the entire story. By going back in time and curing Goku, Trunks ensures his fractured future can no longer happen, undoing the cavalcade of death the Androids caused in his own timeline. However, Future Trunks has appeared many other times in Dragon Ball and has cemented himself as a fan-favorite character. In later games and other side content, Trunks has been turned into a full-on protector of time, leveraging his experiences to put an end to time-traveling threats of all kinds.
The History of Trunks is by far the saddest and most depressing story in Dragon Ball history. However, the young Saiyan's time travel has thankfully rendered it moot. While it is an amazingly written tale, it would feel very anti-climactic for all the show's legendary heroes to go out in such a sad and depressing manner. However, it does show how the Dragon Ball world is perfect for horror, and that Akira Toriyama can write extremely bleak stories. So many fans, while saddened by the History of Trunks, hope the legendary writer returns to this style of storytelling sometime in the future.