The Flash finale shocked us all when Barry ran back in time to save his mother, effectively plunging all the DC TV shows into an uncertain new timeline. This will have huge repercussions in the next seasons of all the shows, but there's another catastrophic timeline change that we all forgot about — the Legends essentially doomed the Earth in their season finale.
For heroes that claim to protect the timeline, the Legends of Tomorrow play very fast and loose with the laws of causality. This is pretty much essential to their concept — we love the Legends because they mean well, but can't stop screwing up their own missions.
Of course, the Time Masters banked on exactly this when using the Legends to ensure Savage's rise to power. But ultimately, Rip Hunter's shortsightedness may have ensured the Earth's destruction at the hands of a vicious alien race.
Savage Is Crucial To Earth's Survival
The last two episodes of Legends of Tomorrow Season 1 threw us plot twist after plot twist, revealing that not only were the Time Masters helping Savage all along, they had carefully manipulated the Legends' actions in order to achieve this goal. Their reasons were sound however: As Zaman Druce revealed to Rip, Savage conquering the Earth is crucial to humanity surviving an alien invasion.
Claiming that there is no other hope for humanity, Druce explained why Savage had to succeed.
"In 2175, less than ten years from when Savage conquers the world, the Earth is attacked by a race of extraterrestrials from the planet Thanagar. Without Savage to unite the world under a singular ruler... all human life is exstinguished."
In typical Legends fashion, Rip promptly leads the team into a battle against Savage and they kill him in three different time periods, once Snart died destroying the Oculus.
Of course, Savage had embarked on his own mission to destroy all time to maximize his power, so the Legends did technically save the world from that eventuality. But does this mean the Earth is doomed to be invaded by Thanagarians?
Humanity's Last Hope
Logically, it follows that without Savage to "unite the world", Earth will be overtaken by Thanagarian forces. Except, we only have Druce's word to trust on this. He assured Rip that the Time Masters examined the timeline for any other possibilities, and came up blank. But they could have missed one vital player in this game: Savage's daughter, Cassandra.
We met Cassandra in the episode "Leviathon", when the Waverider finally travelled to Savage's post-apocalyptic future. Cassandra's role in this episode was really interesting, as she learned her father had caused the plague which decimated the Earth's population, and eventually joined the rebellion against him.
Why is this important for saving the Earth from the Thanagarians? Although all of the Legends' actions were intended to strengthen Savage's hold on Earth, the events of "Leviathan" occurred just before Savage embarked on his mission to destroy time in "Legendary". This leaves Cassandra free from the Time Master's interference.
We don't yet know what the consequences of the finale are, but it could be that with Cassandra's help the rebellion overcomes Savage's remaining forces. Savage trained his daughter on battle strategies and rulership, which means that when the Thanagarians attack ten years later, Cassandra might be able to rally the Earth's forces and defeat them.
Here's hoping we find out about this in Season 2, because otherwise the Legends of Tomorrow just doomed the entire planet!