72-Hour Challenges, also called Card Flipping Events, were events that lasted for, as the name suggests, 72 hours. While shorter versions occurred, the mechanics were the same.
Players earned by completing various tasks. Unlike other events, there was no cap on the amount players could receive with each task. During the final recurrences of the Challenge, challenge coins were earned from a special roulette when players won a Multiplayer battle. All possible results were the same. The amount of challenge coins earned depended on the player's league at the time:
Players spend challenge coins by flipping cards. Players earn rewards with each card flip and, eventually, earn the monster. The cost for each card flip increases each time. Players may also spend to immediately claim the monster. The gem cost reduces with each card flipped.
Despite the appearance that the cards are shuffled, the result of selecting a card is actually predetermined before the card is selected, meaning the card that is selected is irrelevant to the result. It is important to note that the chances of each reward are not necessarily equal. While it is possible to obtain the monster on the first flip, it is far less likely than other rewards. Even when only two cards remain, the chances are not necessarily 50/50.
Most 72-Hour Challenges have 3 monsters that players may attempt to get, listed in order from least to most expensive. After flipping all the cards and collecting all the rewards for each monster, players may (depending on the Challenge) be able to play again.
The following table displays the challenge coin cost for each flip as well as the corresponding gem cost for immediately claiming the prize monster: