The first thing you need to do in this quest is visit The Church of the Brotherhood and retrieve the Bible of Brotherhood from the pews, and then take it to the Two Trees Crossroads and place it on the stone circle there. Then Mathias will appear standing against the tree and give you another key of Nicodemus'. Now go to Nicodemus' House and use the key from Mathias, and then over in the corner behind the barrels you will find a teleportation pad, though cast Reflection before you go through, because you will be ambushed by Skeleton Captains firing Magic Missiles.

Somewhere on level 21 you will find The Chamber of the Wiseman, and when you open the gates the Wizard of the Commandments and a bunch of spectres will attack you. The wizard will drop the Commandment Amulet upon his death.

Next go down to level 22 and go left, and down the end is a switch on the wall. Click on it and put the Commandment Amulet on, and then walk through the doorway back around the corner, by the sign, which should open the gate to a room full of green poison puddles. Keep going and you will come to four gates side by side, you have to close the gate you just came through to open them. Just be careful when walking through them, because you will be attacked by 2 Werewolves. The best way I've found to kill them is to run around behind them into one of the rooms they came from, and then stand in the doorway so you only have to fight one at a time.

After you kill the Werewolves, the gates on the other side will open up to a room full of red chests. Then on the other side of that room there's a corridor leading off to a room with a big stand containing a red Gem. Take the Gem, and then pull the switch next to the gate to open it, and that will lead to a room with 2 nests which spawn wasps. Cast Reflection, and then use Fear to frighten the wasps away from the nests so you can destroy them (go here if you need more help).

After you've finished with the wasps you will find yourself back where you started. This time go to the gate directly in front of the stairs and put the red Gem on the blue plate beside the gate. After you get about halfway down the corridor, the gates on either side will open releasing the wasps, so be ready with Reflection. Kill them and then go down the middle passage leading off to the left (left from where you came in), and you should come into another room with flying wasps. Now go around the other side and through the two gates there, and you will come to another nest and a switch on the other side of it. Destroy the nest and pull the switch, and then go back to the room you came from and the gate at the end of the corridor should have opened up to a room with a black knight standing in the middle. Place the Commandment Amulet on him and be ready for a fight; with both with the knight and the skeleton archers he summons. Also, upon his death the wooden door will open releasing yet more wasps.

Once you've finished with all of these enemies, go through the broken wooden door and there will be two rooms off to each side, the one on the right full of teleportation pads, and the one on the left blocked by a row of teleportation pads. You can just teleport over these if you have the spell, or you can try to find the right pads to get you to the other side.

Keep going and you will find another row of teleportation pads, do the same as last time. There's also another nest. After you have finished with the nest, keep going down the corridor and get ready for yet another nest... the last one. This nest is completely surrounded by teleportation pads, and the only way I've found to destroy it is to use the Teleportation spell to teleport right next to the nest, and then use Fear and hit the nest.

Now go down to level 23 and find The Chamber of the Mad Druid, here you will find a Werewolf and the Mad Druid. Though both are in separate cells so you won't be fighting them both at once. The Mad Druid drops the Skeleton Ring, which is used to get the Skeleton Shield in The Hall of Frustration, though you don't have to get it since all it does it deflect fireballs in a certain room on level 24, and you can just use Reflection instead. So you can just skip all that and go on to level 24 if you wish.

Somewhere down there you will find The Temple, with a Grand Druid and quite a few of his underlings in attendance. You need to kill the Grand Druid, and relieve him of the Prison Key, and then head over to The Druids' Prison and kill the druid guards there. Then put on the Skeleton Shield (or use Reflection) and go through the next room full of crossbows shooting fireballs. Now go to the last cell to the right and you will find Father Nicodemus. He will tell you the renegade druids are planning to take the village of Estheran and have placed one of their men in the church.

So head back up to the Church of the Brotherhood in Omar, and when you enter the impostor will attack you. Finish him off and Father Nicodemus will appear, talk to him and he reveals the hiding place of the Crystal of Integrity - the statue beside him.