It sounds like you're referring to Steins;Gate: Kyoukaimenjou no Missing Link (also known as Steins;Gate: Kyoukaimenjou no Missing Link - Divide By Zero ), the alternate 23rd episode of the original Steins;Gate anime that leads into Steins;Gate 0 .
Thus, Missing Link is the connective tissue—the unseen tragedy—that the original series skipped to reach the happy ending. Steins-Gate- Kyoukaimenjou no Missing Link - Di...
He succeeds in landing on a world line before Kurisu’s death—but at a terrible cost. His Reading Steiner activates, and he realizes he is now in , months after he originally gave up. Kurisu is alive, but… Gate anime that leads into Steins