You need to collect 3 stones in the Defiled Desert. They can be found ONLY after midnight, IF nobody is in area. If people are already in, you have to wait for them to leave, for the area to reset and the 3 stones to appear. Then, when the area is reset, you must hurry to collect the stones as Limas, the wandering halfling, will soon take them.

It is also probable that Limas collects the rocks then drop them again a little later, so if you follow him you might find them too. I never did that way, though.

The stones are normally located in south center part of the area, forming a sort of triangle.

Then, when you have all the 3 stones, you are be able to travel through blue "teleportating" circles that you will find here and there. Note that you need only one stone to travel through circles, but you need all 3 for the spirit to appear later.

Travel randomly through circles till you find yourself in NW corner of area, then enter the 3-stones triangle which lay on the ground. This will automatically start the dialog with the spirit.

If it's the first time you do it, you will also collect some XPs reward.

Note also that if you leave Defiled you will keep the stones in inventory, BUT if you reenter you will LOOSE them.