DO NOT use semicolons and colons interchangeably. Change the semicolon after choices to a colon. (A semicolon has only two uses, neither of which is to introduce a list.)
The comma after slaves isn’t absolutely essential, but it does aid a lot with clarity.
The double quotation marks around benevolent are used correctly, indicating that these are so-called benevolent aliens but aren’t, y’know, actually benevolent. You could also get rid of the double quotes and add a word such as supposedly in front of benevolent.
Just when Earth is about to be destroyed by a huge catastrophe, a fleet of “benevolent” aliens shows up. They give every human two choices: either serve them as soldiers and slaves, or stay on Earth and perish.