So I don't think people see this that much but there's a connection between SF and SM. Remember that gem from Sonic Mania? Well that's the same gem in Sonic Forces. You can tell by the sound effects of the gem in both games. But how did that gem end up in Sonic Forces? Well, simple. In the "true" ending in Sonic Mania when Classic Sonic defeats that robot. He goes through a portal which explains why Classic Sonic is in Sonic Forces because he came through that portal. Now how the gem got there.....It's complicated. Eggman gets the gem in the "true" ending so he could have throwed it in the portal but the portal probably closed before he could. But now we got another thing to help us figure this out. Does anyone remember the portal in the last zone in normal mode and angel island zone in encore mode? Well they used the gem to create that portal so what if eggman opened a portal with it and sended the gem to the modern generation which leads to Sonic Forces. Now remember Infinite the Edge lord? Well he haves that gem on his chest. Modern Eggman used the gem to create Infinite the Edge lord. Which is how Infinite has the illusion ability is because that gem makes it happen.

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BUT HEY! THAT'S JUST A THEORY, A SONIC THEORY! See ya MasterFellows next time!