The light tower is a approximately 30 foot tall tower with a solid center except for a 2 foot wide shaft in the middle. It has no roof and the door in the front leads into a 5'x5' room which the shaft connects to through the ceiling. From the outside it looks honeycombed by many windows arranged in artful patterns, all to small for a man to pass through but they all connect to the internal shaft, also there is no glass in the windows.
There are many theories on what the tower was originally intended for, such as a furnace, kiln, or just decoration. The current use is a cheap and easy way of execution. For an unknown reason the tower, when hit by moonlight emits a bright flash of light, causing the windows to emit light and a column of light to be flung into the sky by the shaft. If anyone is in the room it emits three pulses of light instead of one. (The wizards say that there is “One for the body, one for the spirit, and one for the soul.” They are vague on the distinction between soul and spirit) All that remains in the room is a small pile of ash and the chair. For unknown reasons the chair can not be moved, smashed, burned, or in anyway changed. The door is the same, it can not be left partially open while the flashes are going off, if it is it is immediately slammed shut and anything being used to prop it open disappears. The wizards theorize that the way the tower works is by destroying the soul and essence of the person, rendering them completely destroyed on every plane including the divine one. This type of execution is used on only the most grave offenders and wizards, because you can neither go to hell or heaven instead ceasing to exist. The reason it is used on wizards is because wizards have a tendency to make it back from the dead, either becoming a Lich or a manifestation of the wizard, retaining all of his old powers.
Some possibilities for the history of the tower include:
The sun god built this tower and since has had a argument with the moon god, the light is his pitiful attempt to take back the night. The disintegration/destruction is simply a side effect.
A band of demon-worshipers found the tower and drew many runes inside the room as a trap for strangers. Unfortunately they drew the wrong runes and it only works when moonlight touches it.
A powerful force of evil is using the tower to capture souls for his undead army, its only a coincidence that the government sends the nastiest right to him.
The chair is the old throne of a god, any mortal that sits in the chair for a while is destroyed.
A couple possibilities for plots are:
A character is wrongly (or rightly) convicted and sent to the tower. He has about an hour to either find the cause of disintegration, have his party prove him innocent, or escape.
The assassins guild is angry at so many of their members being killed. The characters must defend the towers against the assassins, becoming prime targets in the process.
An army of souls collected by the tower are pouring into the material plane, the characters must either kill the person controlling them or, destroy the tower. But you can bet where the enemy camp is gonna be......