SUPERGIRL: The Epic Crossover Begins In The New Promo For Season 3, Episode 8: "Crisis On Earth-X, Part 1"

After an exciting hour of Supergirl, which saw a romantic reunion & an evil awakening, it’s time to put everything on the backburner for a week as Kara heads to Earth-1 for a crossover of epic proportions!
Source: COMIC BOOK MOVIES