The use of magic is dangerous and unpredictable. Simple spells sometimes fail or backfire for no apparent reason, while powerful magics are prone to Magical Disasters such as attracting the attention of demons or elemental forces. Used in moderation, however, it can be a powerful tool.
This document discusses magic in campaign terms. For specific GURPS rules, refer to The Demesnes - Variant GURPS Rules and The Demesnes - Variant GURPS Spells.
Mana is not so easily available on the open ocean or high in the air. This means magical flights are either very short or very low, and driving sea monsters or pirates off with magic is a difficult proposition. Spells can be attempted far from land, but the chances of Magical Disaster are high.
Mana can also be released from the spellcaster's own body if necessary. This is tiring however, and if too much mana is used the spellcaster can be injured. Some evil spellcasters will draw mana from other people's bodies.
A trained spellcaster can concentrate for a second and get some idea of how likely a Disaster is when a spell is to be cast. If the danger is deemed too great the spell can be aborted without consequence.
Some areas contain mana which is aspected in various ways. Such an area might make healing magic easier and necromantic magic more difficult, or vice versa. Other aspects, such as fire, water, good, evil, etc are possible. The aspect of mana is often related to the area in which it is found. Lofty mountain peaks might favour air magic, seasides water magic, battlefields death magic, and so on.
The mana to power clerical magic may come from a variety of sources. Often the mana in the earth is used just as for wizards. Some gods supply mana directly via a holy object. Others empower their clerics to draw mana more freely from other sources, such as blood (for war or death gods), or water (for sea gods).
Amongst races of low intelligence, clerical magic may be the only magic available. Goblins may not have wizards, but their shamans have access to dangerous offensive spells!
Some magical items have mana infused into them when they are made, while others merely serve as a conduit to channel ambient mana into a useful form. The difference between these two is important.
Potions and spell scrolls are always stored mana items - their mana runs out after one use. Wands and staves also often store mana, but the item may be used multiple times as long as there is enough stored mana to power an effect. Such items may sometimes be "rechargeable", by gathering more mana and storing it back in the item. The gathering of mana during the creation and recharging of stored mana items is when Magical Disasters may occur. Also, if a stored mana item is broken in such a way as to break its enchantment the mana is released back into the surroundings. A large amount of mana released in this way may cause a Magical Disaster.
Items having magical effects which are apparently always in effect are a special case. Often such items are not really "working" all the time. For example, a sword which magically does more damage than normal is only working magically when it hits. A sword which improves its wielder's skill is only working when it is swung in combat. An amulet which makes its wearer more attractive is only really working when the wearer meets someone. Items such as this use ambient mana only when they really need to. So a magical sword in its scabbard will not cause a Disaster, but one used in a pitched battle might drain enough mana from the area to have some effect. Such effects begin relatively small, but grow larger as more mana is used. The wise adventurer will drop his magical sword at the first sign of any unusual magical effects occuring spontaneously - as hard as this may be in battle!
A few items really are working all the time, such as an amulet which makes the wearer taller. Such items generally have effects which are quite low-powered compared to items which require mana only sporadically. They usually collect mana from the surroundings at a very low rate, which most places are capable of supplying continuously. If such items are however taken into regions with extremely low mana concentrations, Disasters can indeed happen.