Okay, the priest that i got the quest from hints at Shereguen Castle Dungeons, but I've searched there and all I found was a Voodoo Doll and a Bo Stick +1. So how do i complete this quest? And does it have something to do with the rotted corpse I found in one of the dungeon cells? Last thing --- WHAT THE HECK DOES A VOODOO DOLL DO?

From memory, Galians letter (the proof you need) is not in the obvious cellblock area. When you enter the castle, going left you arrive at the room with the painting, and then the samll kitchen like area with the lift down. Opposite the lift in that room, there is a door that leads to about 3 or 4 rooms with skellies in them - in the furtherst right room (north if I recall) you come across Galians letter in a chest. If the directions are out a bit, it will be in that general area. Voodoo doll - don't have one. Does it have any red restriction message when you identify and right click on it. If so, general rule is pass it around and see if someone can invoke or equip it. Let me know if you have any success.

voodoo dolls cast curse when invoked
later in the game you will find a ton of them more or less

Thanks for the warning - do you mean curse the guy that invokes it, or use item to curse someone else in battle (or both depending on if your silly enough to invoke the curse on oneself)?