READ AT OWN RISK!
https://www.youtube.com/watch?v=lHGefDh9xWY (listen to that for dramatic effect)
- Spoiler:
- During the end fight with Dr. Eggman and Robotnik in the death egg, both Sonics go super to defeat them and get back the master emerald,(a callback to Sonic 3) however during the end fight old Sonic realizes what he must do to save the world from the laser that is about to destroy the world and takes the master emerald into the laser's fire port, killing him and destroying the death egg. Sonic then returns to earth and does stuff with the game ending on a cliffhanger as the master emerald crashes down to earth with everyone looking up in the sky.