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Some fans refer to the Season 4 Part 2 premiere as "Episode 5" (relative to the part). This episode shows the beginning of the Rumbling, but the actual reveal of Eren's massive titan skeleton happens immediately after in Episode 80 . in which episode eren become colossal titan
Eren Yeager transforms into a Colossal Titan variant in the final installment of the anime series, titled (alternatively structured as Episode 89 or the final episode of the episodic release). Here is the answer to your question