The Go-Ruby!Sonic AU (or Ruby!Sonic AU, but the former to distinguish from others like Phantom Forces) is one of my earliest Sonic AUs and was inspired by @chacolachao's awesome Ruby!Sonic AU.
To be honest, I wouldn't say it's complete, even after all this time. The ending and middle's really fuzzy, to say the least.
During his 6 months on the Death Egg in Forces, Sonic was tortured by Infinite and his illusions with the powers of the Phantom Ruby. He also struggled to endure being stuck in a cell without his freedom for so long, helping to drive him stir-crazy.
While seemingly not completely off from his normal self, Sonic was ultimately more manic and even sadistic towards his enemies when he was finally freed.
By the end of Sonic Forces, Sonic and Tails found themselves in the possession of the Phantom Ruby.
Months after Forces, the events of the AU start. After having had another nightmare of Infinite's torture, Sonic woke up late one night. In an attempt to get back at the thoughts and memories haunting him, he decided to go into Tails' lab and take out the Phantom Ruby from its containment, despite Tails' concerns and warnings against it. As he was holding the ruby, reality suddenly glitched and Sonic felt a piercing pain through his chest, as if he had been stabbed. He couldn't help but yell and fell unconscious.
Sonic woke up the next morning, being monitored by a worried Tails.
Looking down at his chest, he found it. The Phantom Ruby, in his chest. No matter how hard he tugged and pulled, it was stuck.
Sonic is the main character of the Go-Ruby Sonic AU.
Though the truth inevitably got out, Sonic first attempted to hide the ruby, though somewhat crudely as he mainly used a bandanna.
Initially, Sonic was desperate to get rid of the ruby. This was compounded as he started to hear and see Infinite, more and more often as time went on, to the point the jackal almost always haunted him.
However, Sonic himself also started to change, becoming more cold, arrogant and embracing the ruby's power as his own. Eventually he became an enemy, to his foes and his former friends.
After his defeat in Sonic Forces, Infinite became trapped inside the Phantom Ruby, the same one that Tails would study and contain. Unfortunately, Infinite's presence would also taint the ruby and its power.
When Sonic became fused with the Phantom Ruby, Infinite became bound to him. While Sonic only heard and saw more of him over time, Infinite was always stuck with him, forced to watch regardless of how unaware his new host was.
Upon becoming more visible to Sonic, Infinite tried to use his very limited power to torment him, or convince him to use the Ruby's power in ways he considered worthwhile. Despite some of his words getting to Sonic, and his presence still being a pain, the hedgehog shrugged off a lot of his attempts. Sometimes he would twist Infinite's prompts, for example, taking Infinite's desire to get revenge on Shadow and using the ruby's power to just mess with him instead.
At first unbenknownst to Infinite himself, Infinite's presence in the ruby started to corrupt Sonic into being like him. Unfortunately, this still wouldn't work out in Infinite's favour, not having any control over Sonic and even coming to backfire as Sonic happily became more cruel towards him.
Tails the Fox is Sonic's closest friend and adoptive brother.
Despite Sonic hoping he'd never even know out about him taking out the ruby, Tails was the first to find out. He tried his best to help Sonic's situation, even with the betrayal of his trust.
In the end when Sonic is incredibly corrupted, he still (and only) cares about Tails.
-I guess as kind of a leftover from my old AU stuff, Sonic lives with Tails in this AU.