The Dragon Ball universe has seen some major changes to the very foundation of their reality over the years. Even the timeline itself has been manipulated multiple times, with one version of Trunks even traveling through it multiple times to try and salvage his world in Dragon Ball Z. But despite his best efforts, his dark future only got worse and worse. What has all happened in the timeline of Future Trunks, and how did the franchises' answer to "Days of Future Past" get even darker in Dragon Ball Super?
Trunks and the Deaths of the Z-Warriors
The timeline Future Trunks came from was actually the original series of events for the core Dragon Ball timeline, otherwise known as Universe 6. Without Future Trunks to save the Z-Warriors from Frieza and King Cold, the two alien warlords were instead defeated by Goku -- who sensed the brewing battle and used his newly learned Instant Transmission to reach the planet in time. Content with their victory, the Z-Warriors went back to their lives. However, not long after, Goku was felled by a surprising enemy: heart disease.
The warrior's heart gave out, and Goku passed away of natural causes -- meaning he wasn't able to be revived by the Dragon Balls. The heroes dealt with this tragedy and tried to continue on without him -- up until the arrival of Android 17 and Android 18 a few years later. The Androids were far more powerful than anything the Z-Warriors were prepared for, with Piccolo being an early casualty of the battle. This meant Kame died as well, stripping the Earth of their Dragon Balls. In short order, the rest of Earth's defenders -- Krillin, Yamcha and Tien -- were also felled. Even Vegeta couldn't keep up with Android 17 and Android 18 and was killed despite achieving the Super Saiyan form.
The History of Trunks
Only Gohan, Yajirobe, and Bulma survived, with Bulma giving birth to her and Vegeta's son, Trunks. Android 17 and Android 18 traveled the world, wiping out large swaths of humanity in a series of wild and brash attacks. Gohan did his best to protect the world, spending thirteen years fighting them. He even achieves the Super Saiyan form on his own, and later takes a pre-teen Trunks as his pupil. But without the power boosts training with his father could have provided, Gohan never reaches his true potential. Every battle is a loss, and a particularly brutal one costs him one of his arms. Confronting the Androids alone one last time, Gohan is defeated and killed -- giving Trunks the final push he needs to achieve his own Super Saiyan form.
Bulma eventually succeeds in constructing a time-machine, allowing Trunks to travel back and change the past, starting with the defeat of Frieza and King Cold. This splits the timeline, with Universe 6 continuing on in an altered form. Warning the Z-Warriors of the Androids and giving Goku the medicine he would need to survive his heart disease, Trunks ends up giving the Z-Warriors a chance to survive. After moving to a different part of the past and helping combat Cell and his attempts to reach his Perfect Form, Trunks eventually returns to his timeline with a significant power boost. There, Trunks is able to not only wipe out the Androids but even his timeline's version of Cell, who's never able to achieve anything outside his Imperfect Form.
Goku Black, Zamasu and the End of the World
Peace seemed to return to Trunks' timeline, with him becoming Earth's full protector. He even trained under the Supreme Kai of his timeline, achieving Super Saiyan 2. With this power, he was able to stop Dabura and Babidi before they could revive Majin Buu. However, the Supreme Kai died in the battle, taking this timeline's version of Beerus with him. This meant that when versions of Zamasu (including Goku Black) came to his time, they didn't have to deal with the Supreme Kai. This allowed Goku Black to help Future Zamasu gain immortality and begin the extermination of all humanity. Bulma attempts to use the Time Machine to bring the past Z-Warriors as reinforcements, but is killed by Goku Black before she can get aboard. Instead, it's Trunks who takes the time machine and reaches the Z-Warriors.
Goku, Vegeta, and eventually Bulma join him in the future and help fight against Zamasu and Goku Black, although the sheer power of their enemies forces the heroes to constantly rethink their offense. Eventually unleashing every plan they can conceive of, Zamasu and Goku Black are defeated, ultimately requiring the core-universe Beerus and Whis to completely wipe them out. In the process, every living thing still in that timeline -- save for the trio from the core timeline, Trunks and his world's version of Mai -- is wiped from existence.
Whis and Beerus are able to make a copy of the timeline where Goku Black wasn't able to wipe out humanity. While this is considered a happy enough ending for Trunks and Mai -- who make homes for themselves in this new copy reality -- it still means they're the only survivors of the very original Dragon Ball timeline.