One common type of traditional magic is foretelling the future, which causes a problem analogous to changing the past in time travel campaigns. Read chapter 3 of GURPS Time Travel, decide what sort of time you will use, tell the players before character creation, and stick to it. Ultimately the problem is if the future is highly mutable and/or divinations are uselessly vague, the PCs are cheated of any points paid for the ability. But if the future is rigid, the GM often must force character actions to make it happen, and the players may wonder why they need come to the game. Some problem players may also try to thwart the future just to 'beat' the GM. It is tempting to twist in-game reality to force events, but don't; simply tell the players no up front and go on. In the long run useful divination requires a certain amount of railroading, and it is better to do it openly and out of character than to break the campaign trying to force it in-game.
One other common pitfall - never twist the meaning of the words of a prophecy. Yes it is common in tales, and yes it is very tempting since language is flexible enough you can always do it, but in addition to cheating the PC of the points, it is unrealistic unless the prophecy actually was delivered verbally in the language you give it to the players in.
The Path of Visions allows most divination effects, but divination rituals are often though of as distinct from other magical procedures
This advantage allows the character to learn any effective form of divination. It replaces the assorted advantages allowing use of the GURPS Magic Divination spell, specifically Divination Talent (CI36) and older forms of Blessed.
The skill of obtaining information from divination. While determining the future is a common application, most form of divination can answer questions about the past or present too, and often more clearly.
There are many versions, astrology (through the study of the stars), augury (by examination of entrails), and oineromancy (interpreting dreams or hallucinations) are probably the oldest. Other common methods include observing shapes or patterns in anything (fire, clouds, the motions of birds, molten metal poured into water, heat cracked tortise shells) and reading patterns in the fall of objects (arrows, stones, flour, sticks, straws, dice, coins). In literate societies systems associated with letters (drawing runes from a bag, ouija boards, symbol marked dice, opening a book at random) are also popular. In the modern world card reading is common, but appears to be a Renaissance innovation. See the Divination spell (M48, G59 and Roleplayer 21) for some possible methods.
Most forms of divination take 10 minutes to a couple of hours to prepare and interpret. The player frames his question, and the GM makes the die roll in secret. On a successful roll he should reveal some useful information related to the question, on a normal failure the signs are unclear, on a critical failure the GM lies. Immediate repeat attempts always produce unclear results.
Some forms of divination are suited only to particular sorts of questions - divination by tossing a coin requires a yes or no question, astrology is better suited to durable trends since the stars change slowly, while sensitive processes like flames or the fall pebbles work better for more localized or transient events (are the spirits angry? does she love me?). You can try to ask other questions, but the GM should keep the limitations and strengths of the method in mind when formulating the answers provided.
Much of what magicians do revolves around their ability to know things and sense connections unavailable to ordinary mortals. Sometimes this is through Divination, but many mages have uncanny sensory abilities. Common Sense(B19), Danger Sense(B20), Empathy(B20) and Intuition(B20) are all fairly standard, as are many other sensory advantages, Second Sight and Cinematic Blindness.
Senses are methods of obtaining information about the environment.
The base cost depends on how common and informative the signals it detects are. It is also the cost for a level of Acute [sense].
The effective range of a sense is the distance at which it will detect a human scale target on an Alertness roll. Multiply the base cost by the modifier for that distance from the Speed/Range Table +3, or see the following table. For comparison purposes, human vision has a 100 hex range, hearing 10 hexes, and smell about 1 hex.
|C (under 1 hex)||2||4.5||7||300||18|
Standard senses localize a signal source anywhere in range within inches, good enough to aim a weapon at it. Lesser abilities are treated as limitations:
Limited Arc (-33%): The sense doesn't cover the entire sphere, it has blind arcs. Human vision has this limitation.
No Targeting (-33%): The sense provides the general position of the source, but is not accurate enough to allow aiming. Human hearing has this limitation.
Non-local (-67%): The sense provides almost no directional or range information at all, the best you can usually do is to know the source is around here somewhere. The human sense of smell has this limitation.
Sensory abilities in GURPS have traditionally been priced arbitrarily, which makes it difficult to compare them, but these base costs are close to the more common senses: Vision or equivalent (no color) (4), Color Vision (5), 'Dark' Vision (independent of light source) (8), 'Active' Vision (Radar, sonar etc. independent of light, but you emit something) (6), Hearing (3), Broad Spectrum Hearing (4).
Senses humans don't depend on are harder to price. I think Smell/Taste (5) is about right. Some species have significant ranges, though since currents move odors unpredictably at least the No Targeting limitation may be unavoidable. For Touch (including pressure, pain, warmth and vibration, humans have it at mixed Touch/C range)(3), Sensitive Touch (5).
Inner Sight (10) is the common psi ability to see anything in range, ignoring light or obstructions, and examine it close up from any angle, including from the inside. It is equivalent to Sense of Perception (CI p00), but that sense is severely under-priced. Magicians sometimes have it at ranges of a few hexes.