Palpatine's Secret Apprentice
Vader cloned after Mustafar
Following Vader's near-death and crippling on Mustafar, Palpatine devised a diabolical plot of cloning Vader. During the rushed reconstruction process that saved his life, Palpatine had his medic droids extract the necessary materials and genetics, sending them to Kamino with instructions to clone the subject. In the following weeks and months, Vader needed to have regular medical attention and check-ups in order to ensure his continued survival. And each time he visited, yet more genetic material was extracted.
After several dozen failures, several of the infant clones was safely stabilized and 'born'. The clones (dubbed 'Sabers' by Sidious) spent five years being raised on Kamino. While there, one of the clones retained enough of Vader's memories to name himself (secretly) Anakin. All of the clones became quickly aware of the Force and how to interact with it. Unlike his emotionally-unstable template, Anakin was very devoted to the soothing sensations he felt when he entered the 'nice side' of the Force. But he was no stranger to the dark side, giving into anger whenever some of his 'siblings' started purposely provoking him and using it to defend himself and drive his attackers off.
Upon their tenth birthdays, the Sabers were given a test by their 'father'. He decided it was time to personally begin training the best of them in the ways of the dark side. Sidious visited Kamino and supervised their tests. He quickly noticed one of the clones was unusually attuned to the light. He 'randomly' chose that clone to fight a no-holds-barred brawl against the single most promising of the dark clones. Although the fight was bloody, the dark clone triumphed and beat Anakin to near-death. Sidious proudly proclaimed this clone his apprentice, gave him a lightsaber and ordered him to prove his worth by massacring his remaining brothers. The clone did so, happily. They departed Kamino for one of the Emperor's private resorts to begin 'Saber's training.
Anakin was so close to dying that even Sidious assumed he'd died. But Anakin had been able to enter into a premature form of a healing trance and nursed his body back into health just enough for him to escape being cremated and disposed of. Hiding in the storage units while continuing his healing trance, Anakin slowly recovered his strength. He decided that he wanted nothing to do with Palpatine, his 'brother', or the Empire. Once he was strong enough, Anakin stowed aboard one of the vessels departing Kamino and headed out into the galaxy to begin his own life, free of Palpatine and his Empire.