So you've got your Sadistic Choice (and hey look, I decided). The fate of the world -- which obviously rests in Erasmo's hands -- is to either become slaves forever to the evil Biebots, or else rip a hole in the space-time continuum, thus destroying the Biebots but also humanity as we know it. How do you, the author, decide what he does?
First, there is no right or wrong answer, but there are potential pitfalls which we'll get to in a second. Like everything in writing, what matters is not so much what you do, but how.
Erasmo might actually choose one or the other. He might opt to become slaves, hoping for a future where they can throw off their oppressors (and leaving room for more books). He might opt for self-annihilation, leaving the reader to ponder big questions about life and existence.
But what if you want a happy ending? Then you do what thousands and billions of storytellers have done before you: you have Erasmo take a Third Option. This Third Option can be almost anything, but there are some pitfalls you should avoid.
PITFALL #1: Deus-Ex Machina. In which the author pulls a Third Option out of their butt. Like if a second alien race -- that has been at war with the Biebots for millenia, but we've only heard about them just at the climax -- swoops in and saves the day. Happy Ending, Sad Reader.
PITFALL #2: Why Didn't He Do That in the First Place? In which the reader wonders why Erasmo didn't just do that the whole time, and why the conflict was a conflict at all, and why they wasted their time with the story. Like if Erasmo had a massive EMP bomb in his garage that would shut down the Biebots permanently. He had it the whole time, but arbitrarily noticed it only at the climax.
PITFALL #3: Underestimating the Reader. The moment you present a Sadistic Choice, the reader will be looking for a Third Option. If there's an obvious one that Erasmo doesn't try or at least address ("I have an EMP bomb, but it doesn't work on them. We tried that back in The War."), they'll decide Erasmo is dumb and not worth their sympathy.
Again, this is all subjective. A Deus-Ex Machina can be managed by foreshadowing ahead of time (maybe Erasmo tries to find the second alien race earlier in the novel, but fails), but even then some readers might complain.
I can't think of a better ending to this post, so as a cop-out, here's Joey Tribiani's take on the Third Option.