Currently there are 24 quest zones 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
"If you completed the zones prior to achievements being released then you will have to redo the NM portion of the zone again to get credit. To reset a zone, select the difficulty tab you want to reset and click the Reset button."
All items (28)