The Draw Power spell in GURPS Grimoire is introduced with little fanfare, but its implications are profound. Including it in the list of spells to which most wizards have access will change the way mage characters operate, and it should have profound effects on the behavior of NPC wizards, and what they can accomplish.
The real key to seeing the implications is in the fourth paragraph of the Conduct Power spell. "The low-TL versions of this spell allow the mage to tap natural powerhouses, such as waterfalls, sunshine, the wind, forest fires, volcanoes . . ." It can be assumed that low-TL versions of Draw Power can accomplish the same thing.
Which natural power sources would be useful to the low-tech mage? Sunlight and wind are relatively constant and dependable, but diffuse . . . you would need to gather the light from the area of several football fields to get the two megawatts that is needed to get the spell to work on a second-to-second basis. A gamemaster might rule that, say, a sixtieth of that area would produce 1 fatigue per second, but that's not terribly useful. And the spell isn't an area spell; how one would use it to gather sunlight or wind over a large area isn't clear. All in all, sunlight and wind directly aren't much of a power source. Even concentrating the power with a windmill doesn't help; the largest modern wind turbines only gather a few kilowatts.
Geothermal power is much more energetic and concentrated. Geysers would make a good power source, generating between 2 and 100 megawatts depending on their size. The drawbacks to them is that they aren't constant, and they're a rather fragile system. You could trigger an eruption with the Geyser spell, but if you did this too often you might shut the geyser down completely. Earthquakes in Yellowstone, measuring only 2-3 on the Richter scale, regularly cause geysers to start, stop or alter their schedules. Most geysers only erupt for a few seconds at a time, so the spells that could be cast with it are limited.
Volcanoes are less fragile, but difficult to stay close to for any length of time. Even a small one will generate enough power for the largest circle of mages, but the enchantments required to keep the mages alive while they're in range of the spell would take a lot of preparation.
Forest fires also release a lot of energy, but using one's energy to fuel sorcery is not something a mage could plan on for the long term. It would make a devastating combination when besieging a forest stronghold, however. About 10 hexes of forest fire should provide roughly a megawatt of power, if it's burning strongly. Grass fires would need an area 10 times as large. Chances are, however, if you can see the fire and you're close enough to use it, you're probably close enough to take damage from the heat radiating from it (1d-3 or so per round).
Similarly, a thunderstorm can be drawn upon for up to 100 megawatts for a small storm, 400 for a moderate storm or 1,000 or more for a tropical storm. A hurricane goes off the scale; even the largest circle of mages could not drain a hurricane significantly, even if they were to stand outside in the middle of one. Storms have similar drawbacks to forest fires; as they get more energetic, the less one wants to stand around casting spells in the middle of it, and one can't be sure it will be there when it is needed.
Waterfalls provide a good combination of constancy, power and reliability. A large waterfall like Niagara or Victoria Falls would provide well over 2,000 megawatts, enough for a large circle of mages, and falls of a more common size would provide 200 megawatts, enough for a smaller circle. Small falls, the kind one could find in any mountainous region with moderate rainfall, would provide between 2 and 20 megawatts, well in a useful range. One of the few drawbacks to a waterfall is that an ingenious enemy could stop or slow the flow of water from upstream, whereas a volcano would be more difficult to interdict.
While we set up camp, Teruvio clambered across the rocks next to the waterfall, disappearing into the spray. I stopped to watch. Slowly, the spray cleared, and the turbulent pool at the bottom of the falls turned still as glass. The thunder that had been deafening us for the past hour faded away. The water spilling over the ledge took on an eerie cast, flowing cleanly and gently into the glass-smooth pool, with nary a ripple.
The effects of drawing power out of any natural source will dampen that source, to an extent that depends on what fraction of its power is being drawn. Drawing on sunlight or wind will cause darkness or still air in the region from which the energy is drawn - though the darkness is not likely to be terribly deep, as ambient light from nearby will illuminate it sufficiently for most purposes.
A geyser eruption will emerge as a gentle flow of unpressurized water of ambient temperature. Because the abrupt drop in temperature will cause mineral deposits to form on the inside of the geyser, the geyser will become dormant for 3d months if a 6- is rolled on 3d, -1 per consecutive time the geyser is drawn upon.
In the unlikely event that a majority of a volcano's eruption power is drawn upon, a dangerous condition may be formed where a plug of cool hard material is formed in its throat. This may well cause the volcano to explode (remember Mount St. Helens?).
A forest fire drawn upon by more than a small fraction will grow more slowly, or start to shrink. If drawn upon by more than 50 percent it may be extinguished quickly. As a full-fledged forest fire can cover several acres, however, generating thousands of megawatts of power, this is unlikely on the large scale.
A thunderstorm that is drawn upon for more than a small fraction of its power will begin to lessen in size and intensity. Tapping it for more than half its energy will cause it to shrink quickly, as the energy needed to maintain the activity is leeched away.
A waterfall that is drawn upon by more than 50 percent will show the effects mentioned above, as the energy that had gone to making sound, turbulence and eroding the rocks is instead turned to sorcery. This makes waterfalls an even better choice.
Teruvio stood at the water's edge, his hands extended over the still pool, as if hauling upwards on a weight at arm's length. The stillness of the air carried his words to me, but as usual when he was working his sorceries, I understood nothing. Out in the lake, a few yards from where he was standing, a platform of soggy clay about five yards in diameter rose to the surface. Teruvio's hands turned over, and he said something else, and the clay turned to stone. A walkway appeared similarly, and Teruvio moved out onto it as a circle of stone formed around the edge of the platform, slotted into a groove he had made there. By the time we had the camp set up, he had called a 40-foot tower of unmortared stone into existence, with a slate roof, windows and a doorway facing away from the spray. A long, narrow bridge spanned the pool from the doorway to the shore. As it was completed, the waterfall returned to normal with a crash.
Create Earth, Shape Earth and Earth to Stone spells are probably some of the more useful things one can cast with such a huge source of mana. Over a period of several days, an unmolested mage could easily build himself a castle. Essential Earth (GURPS Grimoire, p. 22) would make this castle even more secure.
Area spells in general are a great way to spend large amounts of energy. Bless Crops would make it easier for a town growing in infertile mountain terrain to prosper . . . though the Create Food spell could make it unnecessary at all.
Other more-or-less permanent spells with large casting costs would benefit from this arrangement, such as the more advanced healing spells. Spells with large continuing costs, such as Communication (Grimoire, p. 20) and Telecast (p. Grimoire, p. 73) would also be enormously useful with a continuing source of power.
Enchantment using the power from a Draw Power spell is a difficult proposition, requiring access to at least 60 ST, but several partners using Lend Strength and tapping the same source simultaneously while in circle with the enchanter could be able to manage that. This isn't terribly efficient, however, as the circle could just as easily be used for the enchantment itself, directly. Anyone who can learn Draw Power can learn Enchant as well.
A mage with access to this kind of power would be at a huge advantage in any combat. Within his stronghold, a mage could protect himself and his companions with magical walls both large and strong; he could cast hugely offensive spells with a minimal cost to himself, and summon or create large numbers of creatures to fight on his behalf. With Maintain Spell (Grimoire, p. 72), he needn't even worry about having too many spells "on"!
Once the word got out that we had found a large, uninhabited waterfall, we started getting visits from other mages trying to replace Teruvio. Most of them tried to stop up the flow upstream, one way or another, but between our patrols and Teruvio's magical guardians, they were never all that much of a danger. The ones we had to worry about were the ones who came pretending to want to become apprentices . . .
There are many ways a "plugged-in" mage could be the center of an adventure. The players could be exploring frontier areas looking for usable sites. They could be trying to steal a site from an established mage. They could be servants and apprentices, as these mages (in a world where the Draw Power spell is well known) would need support to fend off other mages.
A whole community of mages might grow up around a river that makes several waterfalls along a course of rapids, or a geyser basin. They would be somewhat isolated, but would also have to cooperate in order to avoid damaging the whole system. This sort of enforced cooperation makes a good backdrop for adventures with a cloak-and-dagger feel.
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