You are correct. I'll post a copied summary someone who played every AC game made since it's a mess if you haven't.
Juno is a ***** who tried to take over the world after the first flare so they sealed her away. They also made a magic ball that would shield the entire earth but Juno used ***** writing to rig to to let her go upon its usage. Upon noticing that, they looked for another solution. Didn't work. Minerva's warning is that using the ball will free Juno while shielding the world, Desmond does it anyway and Juno takes over the world.
http://ofnoobsandmen.com/ <----Good comic Game contact information in profile.