In this AU Eggman got his fat little hands on the Master Emerald, and he used it to create a machine that could let him travel between alternative worlds, but the machine malfunctioned...

The machine transported evil spirits from an other world that posessed our main characters, turning them into blood thirsty monsters.
Sonic, Amy, Knuckles, Blaze, Silver, Eggman, and Cream (for some reason) turned into evil versions of themselves.

But not to worry as Tails is here to save the day!

After going trough hell, he finds a way to turn his friends back to normal at the cost of the Chaos Emeralds.

Everything is normal again... except... not really...

Our main characters while forgot what happened, they kept some aspects of their monster selves, Sonic for example became clumsy and has problems controlling himself.
Later he finds out this is because of his evil self wanting to take control of him.

But despite these signs, the group tries to avoid the fact that they are not fine, and carry on with their lives.

But when Eggman starts to notice that things are different, he starts to investigate.
After finding out that Chaos Emerald readings come from within the creatures themselves, He visits the second smartest person on the planet, Tails, to talk about the things he found, but when he starts explaining to him, Tails reveals the truth about the Chaos Emeralds, and that he has problems with keeping his friends from transforming back.

So the two join forces to find a way out...
The group gets a little suspicious about Eggman suddenly siding with them, but they have bigger problems...

Like why does Cream has a knife??!

(Vote if you want to hear more)