Question: What episode of legends of tomorrow is the crossover?
Disambiguated questions and answers: question: What episode of legends of tomorrow is the "Invasion!" crossover?
What episode of legends of tomorrow is the "Crisis on Earth-X, Part 4" crossover?
What episode of legends of tomorrow is the "Crisis on Infinite Earths: Part Five" crossover?
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Natural Questions answer: Invasion ! (from the Wikipedia article "Invasion! (Arrowverse)")