DC Arrowverse Crossover TV Event 

"Crisis on Earth-X" is a major crossover event that occurred in the Arrowverse, the shared universe of superhero television series based on DC Comics characters. It involved the four main Arrowverse shows at the time: "Arrow," "The Flash," "Supergirl," and "Legends of Tomorrow." The crossover aired over two nights in November 2017.

In "Crisis on Earth-X," the heroes of Earth-1 gather for the wedding of Barry Allen (The Flash) and Iris West. However, the festivities are interrupted by the invasion of Earth-X, a parallel Earth where Germany won World War II and where doppelgangers of several Arrowverse heroes exist as villains. The heroes from Earth-1 must team up with counterparts from Earth-38 (where "Supergirl" takes place) and allies from across the multiverse to stop the threat posed by the Nazis of Earth-X.

"Crisis on Earth-X" is widely regarded as one of the most ambitious and successful crossovers in the Arrowverse.

"Elseworlds" is another major crossover event in the Arrowverse, which aired in December 2018. Similar to previous crossovers, it spanned episodes across the main Arrowverse shows: "Arrow," "The Flash," and "Supergirl."

In "Elseworlds," the storyline revolves around Barry Allen (The Flash) and Oliver Queen (Green Arrow) swapping lives due to the machinations of the Monitor, a powerful cosmic being. This switch leads to confusion and chaos as Barry finds himself in Oliver's life and vice versa. Additionally, the characters encounter a reality-bending doctor named John Deegan, who alters reality using a mysterious book known as the Book of Destiny.

Throughout the crossover, Barry, Oliver, and Kara Danvers (Supergirl) must work together to unravel the mystery behind the changes to reality and stop Deegan's plans before they have permanent consequences. Along the way, they encounter alternate versions of familiar characters and face formidable challenges.

"Elseworlds" introduced audiences to Batwoman, played by Ruby Rose, and Gotham City, setting the stage for the subsequent Batwoman series. The crossover also teased the impending crisis that would be central to the following year's crossover event, "Crisis on Infinite Earths."