If Galadriel had joined Sauron, it would have had a significant impact on Middle-earth. Galadriel was Saurons greatest threat in the Second Age, and if she had fallen, those who bore the rings would have been corrupted and enslaved to Saurons will, including Galadriel herself. In the Rings of Power series, Galadriel was presented with the choice of joining forces with Halbrand, who was actually Sauron in disguise, to rule Middle-earth together. She refused, but if she had accepted, their union could have resulted in either peace or destruction, with one converting the other to their own mission.

It is unlikely that Sauron would have changed and turned good if Galadriel were to join him. The Ainur were created to have predisposed virtues, and Saurons desire for power would have remained. If Galadriel had joined Sauron, the forging of the rings would have taken place under Saurons direct supervision, and he would have been able to obtain them all. Galadriel would have proved a particularly useful ally, with her power and knowledge at his side, it would have been all too easy for Sauron to tear apart Middle-earth.