She's Not Crazy

There didn't seem to be much hope for Rena, but Luke was willing to try anything. He'd been trying for three whole years, but she never seemed to stay in one place long enough for him to find her. Now, he was ready.

Leia found out that she was on Mustafar, not too far from where they were. So Han and Chewie flew the Falcon and landed on the broken ground, which didn't seem stable enough to walk on, but Luke was willing to take that risk. He saw a girl, but she didn't look like the Rena he knew. There was a scar across her face, and she carried a red lightsaber. Her hair was a mess, and her face seemed to be permanently glaring.

"Rena..." Luke ran towards her, despite Leia's protests. Before Luke even reached her, Rena held her lightsaber up and swung toward him.
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Takes place right after Return of the Jedi, if Vader and the Emperor had lived and the Alliance lost the war.