The Qeynos Catacombs is the vast ancient water supply to the city of Qeynos. The area was long ago walled off when it was no longer used, but then became the haven of the seedier elements of area, including the beggars and thieves, smugglers and thugs, and up to the members who worship the Plague Bringer himself.
The Qeynos Catacombs can be considered to be an early exercise in factions and how they play a role in the safety of an area. Many of the creatures here (in fact, most) start out ambivalent to most people, and this includes everything but the undead and the people in the Temple of Bertoxxulous, who will both kill anyone that doesn't pass as "evil" very quickly. Then, depending on who and what you decide to pick battles with, various members of the surroundings, be they the guards of Qeynos or the rougher elements, will become aggressive to you. Be aware that killing guards down here, where they are very weak, can lead to you being attacked by the vastly stronger guards in the city above.
Otherwise, the only wandering monsters that are aggressive to all are the large piranha, which can hit fast and for quite a bit of damage to a relatively new adventurer. Take a look in the water before making a long swim through a corridor with no exits, especially if you are not a good swimmer, or you could be trying to get your corpse out of the bottom of the water down here, while avoiding the same piranha.
Recently, the catacombs have become much more dangerous, and several powerful undead spectres can be seen in various areas, as well as wandering septic rats that are very powerful and aggressive. It is therefore wise to be much more careful when travelling around the catacombs than in days past.
This area is a pretty good area for low level adventurers to hunt in and fight in, with lots of readily available creatures that, for the most part, don't group up when you fight them. There is also a good progression up through the levels, and you could fight here from level one through to level ten or so without leaving for anything other than supplies. This is especially true if you are an evil character, for there is a bank located in the Temple of Bertoxxulous as well as merchants that sell all manner of goods.
The additional benefit, in my book, is that this zone is always very uncrowded. There are never more than ten people in the whole zone, and often many many less. This means that you will likely never have to deal with kill-stealing and the other annoying things that happen in the more crowded newbie areas up where the sun is. It is also nice down here if you are evil, because it gives you the feeling of being truly evil, wandering through the sewers beneath the city above.
Travelling To and From Qeynos Catacombs
There are multiple separate ways into the Qeynos Catacombs, all are through tunnels that lead into them, and most of these are underwater ones. It is also possible to pass completely through Qeynos from the outside to where the ship lands by travelling through the Catacombs. The first entrance is located outside the city gates through a secret opening in the walls. Look for the heavy ivy clinging to the wall and the moisture that comes up from the well inside. This is the only entrance from outside the city. Next you are able to enter through the pool outside of the Monk's Guild Hall, by swimming down through a shaft inside. There is also an entrance from underneath Crow's Tavern in the Thieves Guild Hall, but this is unsafe as secret pits will drop you into the shark pit.
There are two entrances near the bank area, one is located behind the bank in the water before the magic-users guild halls. The other is in front of the bank in the water as well. The other underwater entrance lies in the harbor, in the next small area near to the dock where the boat arrives at. The final entrance is the secret entrance that the members of the Temple of Bertoxxulous use to get around, and that is through a secret tunnel lying under the Grounds of Fate.