Arc of Destiny is one of the many rare consumables in Tarisland. It is used to trade for other mounts and apparitions, such as cosmetics, in the Reward of Destiny Redeem section.
Tarisland: Trial Ground of Destiny guide!
Since it's a fairly valuable consumable in Tarisland, it can be tricky (and expensive) to obtain, especially if you want to quickly collect a significant amount.
The only way to get Arc of Destiny in Tarisland is by pulling duplicate items through the Reward of Destiny: Pray gacha system.
When a duplicate of an item is drawn in the Reward of Destiny: Pray system, it will be converted into multiple Arc of Destiny. Different duplicate appearances will be converted into different amounts of Arc of Destiny.