Complete them to unlock powerful new secret runewords - the Fragments of Chaos. A challenge completed by anyone on the Ladder will reveal the runeword for everyone! Therefore you are allowed and even encouraged to trade, borrow and group with other players for all challenges (apart from one).
Additionally, the first person or persons to complete a challenge will be inducted into the Season 13 Hall of Fame once all challenges are finished.
Completion Requirements in Detail
So you want to complete one of the challenges? Here's what you need to do to claim the reward.
► Show Spoiler
To be included in the Hall of Fame
Once all challenges are completed
Immortal: LHawkins1, Darkrai, FuriousBEAR, Highology, unholypal
Forsaken: vampyra, ben-, o_owningtime, annihilus5, seekers
Eternal: Ky0k3n, seekers, aaron-, vampyra, ben-