Dergiab's Head - NWN Side Quest (Chapter 2)
North Road: Dergiab's Head is a side quest that begins by speaking with Gerrol on the North Road map. You'll find Gerrol just outside the gates leading in and out of Port Llast; talking to him will begin both this quest as well as North Road: Gerrol's Wife. Both of these quests take place in more or less the same area and should be completed at the same time.
For this quest you're tasked with eliminating Dergiab who is found in the Ogre Caves map. You can reach the Ogre Caves map by traveling through the Bugbear Caves, which is also the same map you'll find Leah on, Gerrol's Wife. In the eastern portion of the Bugbear Caves map you'll find stairs down that'll lead you to the Ogre Caves.
Dergiab is found in the middle of the Ogre Caves map and he's got some friends with him. One of the people with him is Ganon who is involved in The Search for the Cult, this is one of the main quests of Chapter 2. Be sure to loot Ganon's Journal from the chest in this area and loot Dergiab's Head for this quest.
Once you've defeated Dergiab and obtained Dergiab's Head you'll want to return to Gerrol on the North Road map to turn the quest in. A tip for you, in the room behind Dergiab's Lair you'll find a Portal which will teleport you back to the surface when you use it.