Currently there are 18 quest zones on Mobile with 1-5 areas within to complete on normal difficulty before the next area unlocks, completing an area will also unlock the next difficulty from Normal to Nightmare
Completing a boss on nightmare difficulty will sometimes yield a unique item examples are, the Shadowknight equipment set, crafting components like the Shield of Ryndor
To reset a quest zone follow these steps:
1. Make sure you are in the Zone and Area that you wish to reset and that you are on the correct difficulty.
2. Tap on the area name.
3. Tap "Reset Area", and confirm.