The Challenge Book, which can be found under "Player>Books>Challenge", is a public page that allows you to view which Challenges you or another player has completed. If the page is yours, you will be presented links that let you prepare to try them.
The following restrictions may or may not apply to a particular Challenge:
Creatures of any category can participate in challenges if you purchase the Credit Shop Key ItemGolden Skull.
Players may be inflicted with sicknesses or debuffs at the start of a Challenge. Both conditions can be recovered from with the relevant recovery items or skills needed to do so during a battle.
Enemies can receive a variety of bonuses at the start of the match, such as permanent bonuses to their defense against various sicknesses and powerful buffs.
If one of your creatures is a higher level than an enemy during a challenge, all of the enemies within the challenge will have their levels scaled upwards to compensate.
Not all Challenges are created equally, and some are more valuable to your Challenge Book Completion Percentage than others. You also must complete a Challenge once across all three difficulties to earn all of the points it offers.
Experience rewards are non-existent during Challenges.
You will not lose spirit when you fail challenges.
Show off your book to friends by copying the URL. It's public so anyone can see the difficult battles you've completed!