Unlocking the Entrance
When you have been taken notes, when Lanilor was explaining to you how to find the entrance to the catacombs, this should be easy. If not then go ask him again.... or ..... just turn the four dragon statues to the north. This will make the statue disappear and an entrance going down will show up.

Who also shows up is Lanilor, he gives you a pyramid shape. It is a portal stone that will bring you to the position of the other stone. Lanilor doesn't know where that other stone is as he was afraid to try it out. You will find the other one on your travels through the catacombs. What you could do is drop this pyramid near George's place, so that when you find it you can teleport to him and sell some stuff as you will find a lot of good things to sell. After you sold it to him, you can teleport back to the catacombs again.
Of course you could also use it now, pick up the other one and head back again, but if you do that then you'll find the stone heavily guarded, so you have to be fast in doing that.
Another option would be to put teh stone near a bed so that you could teleport out of the catacombs and take a healing rest.

If by any chance you would unlock the entrance before unfreezing Lanilor then you will get the teleporting stone from Lanilor only at the end of your journey through the catacombs.

Overview of the first level

First Level
You enter the first level and yes this does mean there are more levels, five in total and it will be a hack and slash feast with some required thinking added to it. I'm not going to explain where the baddies are as they are all around you, so take care of where you step.

In the room south of you, you'll find a book, that is the first part of the mystery of the catacombs. You will also find a lever that will open the door on the right hand side of the room. This is common throughout the catacombs (and especially this level).

The Three Orbs
You will be guided to the northern part of the catacombs as one of the doors that could lead you to another direction is closed, so you'll soon find another lever and another door that opens instead. In one of the rooms to the far north of this level you will find an orb, take it with you as it is important. In the room to the right of that you will find a sapphire key, which will open the before mentioned closed door.

An orb

Kill every monster on your path as it allows you to gain experience which will allow you to level up which in turn is needed to be able to do the lowest level. Also look closely behind spiderwebs as they sometimes hold some treasures also.

In the southeast room of this level, you will find another orb. Now go to the west and you'll enter a room with 5 candles in it and the third orb. Take the orb and read the manuscript on the floor. It shows you what happens if you light all 5 candles. And in case you don't get it, just try it out, light the candles (by clicking on the top part of it) and when all 5 are lit a skeleton appears, that you can kill. Nice to gain even more XP I guess.
There is also a book there, read it to learn that it's from someone denounced by the healers of Aleroth and who was working on a resurrection spell. Hit the lever to open the door to the left.

The Orcs
When you go through the door you'll enter a hallway. At the end of it there are several orcs just waiting for you. Now don't panic, they are not there to harm you they want you to do something for them. In order to find out talk to Smiruk. He is missing a magic axe named Slasher and he asks you to go looking for it. That sounds like a quest, so of course you agree.

The friendly orcs

There is also a ladder leading up, but you shouldn't go there now. It leads to the orc camp and if you want to go up, you will find yourself surrounded by orcs and might not survive that right now.
So, say good-bye to the orcs and go south.

An extra pair of hands

This is a big lady

Go down to the most southern room. There is a statue there and a book. Read the book and the sad story of Sylvena and Gregor Brock who was doing a lousy job in protecting her.
When you light the candle the statue will speak and Gregor Brock appears to do his last service as a protector. Or in other words he will be following you around to protect you from all evil. Gregor Brock has no weapons but he can break a skeleton in pieces with his bare hands, so he is a formidable NPC to tag along. However you will also not gain the XP from the kills he makes, so if this is an issue, ditch him. The easiest way to do that is by going back to Aleroth for a short moment, which will terminate him instantly.

The Pentagrams and the Orbs
From the statue go to the north and enter one of the last rooms. In it you will find three pentagrams and another book. Clever as you are you already figured it out of course. Three pentagrams, three orbs, who needs a book to explain that?
So place the orbs on the pentagrams and you will see that a portal appears. Have no fear, just go through it and you are teleported to the room adjacent to this one (which was closed before). In it you'll find two stairways, one leading up and the other leading down. The one leading up brings you to the graveyard of Aleroth. The one leading down brings you to the second level of the catacombs.

Place the orbs on the pentagrams...	...and a gate will appear