Challenges take place on certain days and provide various rewards.

Each Challenge has a different number of missions and rewards. After you complete one of the missions for 3-stars, it automatically becomes available for Instant Loot.
There are specific requirements for some challenges. For example, to take part in the Challenge of Reason, you need to assemble a team of Defender heroes only.
Do not forget to check the requirements for a Challenge by tapping the Book icon in the upper right corner.
To re-participate in the challenge, you are given 3 challenge tickets. Tickets are replenished daily.