Why is that cave on Dagobah tainted? And why is Yoda so determined to stop Luke from challenging Vader? Maybe because Yoda had already fallen to the Dark Side:

Redditor Kavis offers a creative reimagining:

I've thought this for a while now. It makes a lot of sense, I think.

He warned Qui-gon and Obi Wan. The kid was too old, too scarred by a lifetime of slavery. He needed a therapist, not Jedi training. They wouldn't listen. And then the kid, grown into a troubled young man, destroyed everything. Masters, knights, padawans, children. Yoda could feel their deaths echoing through the Force. So he fled to Dagobah, but it didn't stop. The purge burned the Jedi from the galaxy for about twenty years; all the while, Yoda could do nothing but grieve.

Twenty years of misery, loss. Seething and alone in that fetid swamp. The only thing that brought him solace was the cave. He found it early on: a place that resonated with the Force. He meditated there. Poured out his bitterness. Tainted the cave. It's why Luke encountered the phantom Darth Vader there. It was a vision of the man Yoda hated, the man who slowly turned Yoda to the Dark Side. Only what you take with you, indeed.

He drew up his plans to destroy Darth Vader during his exile. He knew the Skywalker children were protected from the purge. He knew Obi Wan would bring them to the swamp for training, in time. Obi Wan never knew Yoda had fallen. Yoda's hatred, his darkness, filled the cave, leaving the old master apparently still standing in the Light. When Luke arrived, it was Yoda's moment of golden opportunity. So he trained Luke.

When Luke left, it was nearly a disaster for Yoda. Luke was unready not because he couldn't possibly defeat Vader, but because defeat was too good for Vader. Yoda needed Vader to be destroyed utterly. To be redeemed, so that Anakin would have time to regret before he died, and so that Darth Vader would be completely annihilated.

The flame of Yoda's hate had burned out his body. He could no longer stave off death with the hope of revenge still to come. When Luke returned, he was ready. Not because he'd somehow become a better duelist or more powerful in the Force in the brief months he was away. No, Luke was ready to turn Vader back to the Light or die trying. So Yoda let go of his hate at last. It was the only thing he had left to live for. He let go of life.

Previously:

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