Nagafen's Lair (informally called Sol B) is a large set of caverns that lie deep in the Lavastorm Mountains. Buried deep inside these caves is the lair of the Fire Giants, who are the personal servants of the mighty dragon, Lord Nagafen. The caverns leading to the dragon are infested with all manner of bats and insects, as well as kobolds that have taken up residence in the entrance to them.
Note: the lettered areas above are zone points to the corresponding lettered areas in Solusek's Eye (Sol A)
This dungeon is generally regarded as one of the two toughest dungeons in all of Antonica. There are many specific dangers that I will describe below, however there are some general dangers as well. First is that everything in this zone sees through invisibility, which means that corpse retrieval and quickly moving through the dungeon are not really possible. You will need to fight your way through everything you meet in order to move further in the dungeon.
The second general danger has to do with the physical layout of the dungeon. The tunnels themselves are winding and confusing, which makes getting lost or getting trapped in a place you don't want to be, as well as a quick getaway, very dangerous. There are zones to Solusek's Eye nearby when you are hunting kobolds, but the path to them is usually very twisty. There also exists within the zone a great deal of lava. People who fall into the lava here will take quite a bit of damage, and although there is usually a nearby place to exit the lava, you have to live until you reach it. Many longtime hunters have entire suits of protection from fire jewelry that they don when hunting here.
Now for specific dangers. The kobolds are probably the easiest to fight in the dungeon, but that's not saying much. They can hit pretty hard and take a lot of damage, and there are a lot of them in the outer areas, so it's recommended to have a group when taking them on.
The "bats and bugs," as they're called, are much more dangerous. The bats themselves aren't too very tough, but they are nearly impossible to solo because of their powerful stun attack, which takes you out of commission for quite a while. The bugs are more dangerous, as many of them have some evil poison- and disease-based DoTs that can wear you out fast. They also tend to travel in packs and aggro easily.
Deeper still lies the Imp Guardians surrounding the Efreeti. This area is very dangerous to hunt in due to the pathing of the creatures, and has a high learning curve for how to do it safely. There is also basically no way to safely flee the area if you run into trouble (other than evac'ing). This is not an easy place to try and gain experience.
Further yet lies the Fire Giants in their temple. The strategy for clearing out the Fire Giants is long and detailed, and requires a highly skilled team of high level players to take on. None of these creatures can be mesmerized at all or charmed except by very high level enchanters, and they will resist all spells if the spellcaster is out of melee range. Knowing the layout of the area is also important, as you will likely aggro other Fire Giants through the walls of the place. Using someone who can feign pull is essential in this area, and it shouldn't even be attempted without it.
The deepest being in this zone is the mighty Lord Nagafen himself. The dragon lies hidden in the deepest part of the dungeon, and is protected by all of the creatures listed above. Taking on the dragon requires the coordinated efforts of several (the least I've heard of is 4) full groups of characters in order to take out, and even then is not something that it done with ease. Having that full suit of fire protection jewelry is only essential here if you want to live...
Having said all of the above, you might be wondering why people would fight here at all. With all of the dangers in this zone, however, come a great deal of both experience and items that are all highly sought after. Before the discovery of the lost continent of Kunark, it was actually difficult to find anything to kill in this zone because it was so crowded. It is now more open for hunters, and you can likely find great experience even during peak times.
There are also several areas within the dungeon where it is relatively safe to hunt creatures without too much danger. Most of these lie near the zones into Solusek's Eye, where it is possibly to pull creatures and, if you are overwhelmed, zone away from them. Mountains of experience can be earned here in a relatively short period of time by a savvy group.
Traveling To and From
There are multiple entrances into Nagafen's Lair. The main one that people take is from the Lavastorm Mountains. Within the mountains, travel to the large lava pit at the north end, known as Solusek's Eye (not to be confused with the zone Solusek's Eye, which everyone calls Sol A). There are two paths to caves on the southern edge of the lava pit, the one you want is the western one, which will take you to Nagafen's Lair.
There are multiple zones from Solusek's Eye (Sol A) into Nagafen's Lair. These are frequently used as escapes from Nagafen's Lair into Sol A, but be aware that they are also used for the same purpose on the other side, and you may be going from the frying pan into the fire when you appear on the other side...