To support the newly-released Justice League movie, Injustice 2 is getting limited time multiverse events, with the rewards being movie-inspired gear.
This is something we have seen before when the Wonder Woman movie hit, giving the Amazonian new gears to equip.
The events can be found in the multiverse and are only there for a limited time. Remember that if you are not good enough with some characters to finish the event, you can setup an AI profile and let them grind for you.