The Mystery Chest is a reward-giving treasure chest that appears at random in AbyssRium, seemingly with no requirements to unlock or spawn. Upon opening the Mystery Chest, you're met with 3 possible rewards in the game, all 3 of which are not worth waiting through the forced ad to get.
The Mystery Chest only seems to feature 3 basic rewards so far, and with no news on whether or not the chest rewards will be updated anytime soon. The current Mystery Chest only rewards either 30 gems, 60 seconds of auto tapping (similar to the skill Volcanic Eruption, but only 5 times per second), or a decent amount of Vitality. The Vitality rewarded from the chest doesn't have any set amount, and appears to only be loosely based around your current earnings.
The only other use for the Mystery Chest, besides the rewards, would be the Moorish Idol, Leatherback Turtle, and Hector Dolphin. The Moorish Idol is the most simple of the three, only requiring 5 pictures of the Mystery Chest to unlock. The Leatherback Turtle is difficult, but still reasonable to achieve if you've got the free time, its requirement being to open the Mystery Chest 50 times to unlock. However, the Hector Dolphin will be a pain to get and takes the longest to achieve, requiring 100 Mystery Chests to be opened in order to unlock! Thankfully though, the chests don't need to be opened all in one sitting for them to count towards the requirements, and will save the progress when you exit the game.