The first part of this quest is done above ground in the land of Omar when you run into Gaeton, who has had his magical axe stolen by Gober the Goblin. Gober's camp will be nearby, but as you get close to it you will be attacked by a hord of goblins (thunder or poison cloud works very well here), once you have killed them, venture further into the camp and you will be attacked by a second wave of goblins. In one of these groups will be Gober, once you kill him he will drop Gaeton's Axe (you will hear a sound like when you find a town quest item). Take this back to Gaeton, and finding you a person worthy of his trust, Gaeton will give you the Shield of Demons. You will need this later.

Now go down to level 21 where you will find a room guarded by a few skeletons, one of which will drop a scroll giving you a clue as to how to walk the road of reason. Once you have gone through the door, you will see the floor is covered by a lot of tiles with rune symbols on them, you have to look at them, and only walk where there are two tiles with the same rune pattern. After you have gained the other side by doing this (don't try to just run across), you'll see a demon statue with a red gem in its hands, the Gem of Reason. Take this but be careful, for as soon as you touch the statue you'll be surrounded by snakes. It's best to have a few healing potions and a good weapon or spell ready (thunder works best if you have it).

Now continue to level 22, where you will find another room, this one guarded by fireballs and Chief Rat Men. Use reflection for the fireballs, and kill the rat men as you normally would, and at the end there's another statue with The Fire Gem, but no ambush this time.

Next go down to level 23, and yet another room, this one guarded by a Giant Worm. Use the switches on either side to open the gate, then it's probably best to use a ranged weapon for killing the worm, and after you kill it a few snakes will appear, so be ready for them too. After that's done, go into the room and down the dark corridor, where you will find the third gem, The Gem of Courage. When you take this you will be ambushed, similar to when taking the first gem, but this time you will have time to run back to the corridor. So break the statue, then run quickly back to the corridor, kill the snakes, then go back for the gem.

Finally, go to level 24 and somewhere on that level will be a room with a stand and the crystal. Place the three gems in the three slots of the stand, and that will open a doorway. Walk through the door and retrieve the helmet there. Equip both this helmet and the Shield of Demons, then you will be able to remove the Crystal of Integrity from the stand.