The video, which comes from Alex Luthor on YouTube, cuts together scenes from both Justice League and Watchmen. Doomsday Clock features the main heroes of both teams and includes cameos from other iterations of the Justice League's main characters, including Grant Gustin's Flash, Christian Bale's Batman, Christopher Reeve's Superman and Lynda Carter's Wonder Woman. The video casts Doctor Manhattan as the film's main antagonist.
This isn't the first time Luthor has created a trailer for a nonexistent film in a comic book cinematic universe. In 2015, he created a fan trailer for Green Lantern Corps and a mashup of Guardians of the Galaxy and Star Wars. The name Doomsday Clock is a reference to DC's ongoing series from Geoff Johns and Gary Frank. That comic sees the characters from Alan Moore and Dave Gibbons' Watchmen entering the DC Universe and the consequences resulting from that introduction.
Both Justice League and Watchmen were directed by Zack Snyder. Watchmen earned over $185 million worldwide against a budget of around $130 million upon its release in 2009. For comparison, Justice League broke $657 million worldwide in 2017, though that movie cost over $300 million to make.
Justice League and Watchmen are both available on home media.