I read this one a long time ago and for the life of my can't find the source. These are the bullet points I remember.

Ben Kenobi is Anakin Skywalker. Darth Vader is Obi-wan Kenobi.

Anakin takes the Kenobi name and goes to his home planet to hide in plain sight

Ben gives a lightsaber to Luke saying "your father wanted you to have this" but we learn later that Vader does not know about Luke.

The Emperor and Vader have a good laugh when discussing Vader's "son." The darkside would surely use the draw of family ties[true of otherwise] to corrupt a young Jedi like Luke.

We know Obi-wan was trained by Qui Gon Jinn, and Ben Kenobi trained by Yoda. If it was Obi-wan that was turned to the darkside Yoda would be the only person left to finish Anakin's training.

When Luke meets Yoda he is told he is to reckless like his father. The ghost of Ben Kenobi says "You said the same thing about me" to which Yoda repeats "Like his father."

There is a point where Vader and Luke have an exchange through the force and Vader calls him "Luke" Luke calls out "Father" and Vader corrects himself and replies "Son"

The battle at the end of Revenge of the Sith is the only thing that ruins this =(