I am seeing three ways Dark Side Ahsoka can work...
Option 01: When she was resurrected on Mortis, the influence of the Son was still planted within her and it slowly grew. The rest of her story in "The Clone Wars" continues as we see up until after the Siege of Mandalore when Anakin falls. The backlash within the Force, followed closely by the clones turning on her with Order 66, pushes her too far and she turns. Her presence is felt and she is hunted by Inquisitors but kills everyone that finds her, proving to be a growing thorn in Palpatine's side. One he is still deciding if it is worth the effort to convert to his side or not.
Option 02: Similar to above but Vader finds her first and she agrees to join him. They work together in secret to take down the Emperor. Not sure if Palps is aware of their conspiracy or not.
Option 03: Anakin's fall happens earlier, during the Siege of Mandalore when Maul is trying his damnedest to tempt Ahsoka. Because of the backlash of Anakin's Fall and the influence of the Son, Ahsoka accepts and the Crimson Dawn gets that much more powerful.
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Option 01: When she was resurrected on Mortis, the influence of the Son was still planted within her and it slowly grew. The rest of her story in "The Clone Wars" continues as we see up until after the Siege of Mandalore when Anakin falls. The backlash within the Force, followed closely by the clones turning on her with Order 66, pushes her too far and she turns. Her presence is felt and she is hunted by Inquisitors but kills everyone that finds her, proving to be a growing thorn in Palpatine's side. One he is still deciding if it is worth the effort to convert to his side or not.
Option 02: Similar to above but Vader finds her first and she agrees to join him. They work together in secret to take down the Emperor. Not sure if Palps is aware of their conspiracy or not.
Option 03: Anakin's fall happens earlier, during the Siege of Mandalore when Maul is trying his damnedest to tempt Ahsoka. Because of the backlash of Anakin's Fall and the influence of the Son, Ahsoka accepts and the Crimson Dawn gets that much more powerful.