Multiverse is a game mode in Injustice 2 where random arcade ladders of AI Opponents are assembled for players to challenge, where thousands of different worlds from different universes appear with their own given storylines. It is up to players to play through each unique premise and conditioning for each fight, and they are usually rewarded with their success that range anywhere from credits and experience to gear and Mother Boxes.
When you first enter Multiverse as a single player, you will see multiple worlds to choose from. Each world is only there for a limited time, and there will be a timer on each world to indicate how long it will be there. After enter each of these worlds, you will see many different challenges for you to battle, each having two separate sets of rewards you can earn. The first reward is a one-time prize that is based on your performance, getting a certain set score during each challenge. The second is a completion reward that can be repeated (though changed each time doing so), based on whether you complete the challenges or not.
Each and every challenge will also show you the suggested Character Level you should have before taking on the challenge itself, as well as what the AI difficulty will be and any other requirements. There are even some challenges that will be locked until you complete another separate challenge, as well as as others that will require you to pay a credit fee. Some challenges will also be what are called Boss Challenges, where you will fight a single enemy with very high stats. With Boss Challenges, there is usually a level or stat restriction that must be met before you can participate; however, these challenge will yield the most rewards.