Od Nua is a mysterious labyrinth build under the fortress, also known as Endless Paths, It has fifteen levels and on each level you will face stronger and more numerous opponents.It is one of more interesting locations in the game. Visiting it (with the exception of several rooms from the first level) is not mandatory and not required to finish the main storyline. Still, you shouldn't ignore this chance. Just make sure that you don't walk there if you have low experience level - enemies will be too strong for you. In Od Nua you will find various references to Dyrwood history and groups that live in it, so at the beginning of the game you won't be able to understand all new information. Virtually every floor has a quest for you, and some of them take even few floors to visit. In addition to various interesting (and strong) enemies, like dragons or ogres, in tunnels of Od Nua you will find many traps, so you should use scouting mode very often. If you give it enough time and attention, you might gather dozens of thousands of coins from chests and found items. Usually each two levels there is a possibility to use Master Staircase that allow you to quickly navigate up and down the Endless Paths.
Important - A detailed description of all fifteen levels of Od Nua (along with high quality maps) can be found in the walkthrough section of this guide.