Crystal Ball I
This is the most common form of scrying device: a crystal sphere about 6 inches in diameter. A wizard can use the device to see over virtually any distance or into other planes of existence. The user of a crystal ball must know the subject to be viewed. Knowledge can be from personal acquaintance, possession of personal belongings, a likeness of the object, or accumulated information. Knowledge, rather than distance, is the key to success at location.
The chance of locating also dictates how long and how frequently a wizard is able to view the subject:
Viewing beyond the periods or frequencies noted force the wizard to roll a saving throw vs. spell each round. A failed saving throw permanently lowers the character's Intelligence by one point and drives the wizard insane until healed.
Certain spells cast upon the user of the crystal ball can improve the chances of using the device successfully. These are comprehend languages, read magic, infravision, and tongues. Two spells - detect magic and detect evil/good - can be cast through a crystal ball. The chance of success is 5% per level of experience of the wizard.
Certain crystal balls have additional powers. These spell functions operate at 10th-level. To determine whether a crystal ball has extra powers, roll percentile dice and consult the following table:
Only creatures with Intelligence of 12 or better have a chance of noticing that they are the subjects of scrying. The base chance is determined by class.
For each point of Intelligence above 12 the creature has an additional arithmetically ascending cumulative chance beginning at 1% (1% at Intelligence 13, 3% at 14, 6% at 15, 10% at 16, 15% at 17, 21% at 18 Intelligence, and so on). These creatures also have a cumulative chance of 1% per level of experience or Hit Dice of detecting scrying. Treat monsters as the group as which they make saving throws. Check each round of scrying, and if the percentage or less is rolled, the subject becomes aware of being watched.
Dispel magic causes a crystal ball to cease functioning for one day. The various protections against crystal ball viewing simply leave the device hazy and nonfunctioning.
The DM may allow other scrying devices for priests and druids - water basins and mirrors are suggested. Have them function as normal crystal balls.