All spellcasting Disciplines in Earthdawn use about the same mechanics. It depends on three Talents: Spellcasting (which can be learned as a skill), Thread Weaving (Elementalism, Illusionism, Nethermancy, Wizardry), and a variety of Spell Matrix talents.
Under normal circumstances, a mage will meditate peacefully for ten minutes to place a spell he knows into one of his matrices ("attune" the matrix). Then he will weave whatever threads are needed to fully power the spell. The difficulty number to weave these threads will vary with the spell, as will the number of threads needed. Each thread will usually require a full action to weave, but an Excellent success on the thread weaving test will convert weaving that thread into a free action. Finally, he will use Spellcasting to launch the spell at its target, with a difficulty number of his target's Spell Defense. The spell's damage will usually be resisted by the target's Mystic armor, but some spells will be resisted by physical armor, and some will ignore all armor. In such cases, the spell's description will explain.
Of course, the mage may not be able to wait ten minutes to attune his matrix. In that case, he will usually re-attune on the fly, which costs one point of damage (strain) for each attempt. Re-attuning on the fly means making a thread weaving test with a difficulty number of the spell's re-attunement number, which is much higher than the normal difficulty to weave threads to the spell. If the mage fails, no harm done -- as long as he keeps trying. Once he gives up the attempt to re-attune on the fly, all of his spell matrices will be wiped clean.
The obvious difficulty here is that mages are quite limited in their number of spell matrix talents. Some mages create spell matrix objects to have more spells on hand. To create a spell matrix object, you must weave a permanent thread to a physical object that has a proper mystical pattern on it (carved, embroidered, or otherwise worked in). The legend point cost is the same as for a first circle talent, and the Thread Weaving difficulty is equal to the current thread rank plus 8. The mage must hold or wear the spell matrix object in order to use it.
Spell matrix objects count as the most basic type of spell matrix. Its permanent thread counts against the limit of permanent threads that any character can have, and it is the only kind of permanent thread available to characters under Fourth Circle. The matrix object can only hold spells of Circle less than its current Thread Rank. If the object is destroyed, the matrix and the spell within it are lost. If the spell matrix is attacked and destroyed (say, from astral space) then the object is also destroyed.
A mage can cast a spell of his Discipline that he does not know, if he has a grimoire with its pattern recorded there. It is more difficult than normal casting, but it can be useful in a crisis. It also is the only way a mage can cast a spell of higher circle than his own. The process is very similar to matrix casting.
First, the mage must attune the spell. He makes the same thread weaving test as he would for re-attuning on the fly, but the difficulty number is at +2. Second, he must weave any threads that the spell requires. However, he faces a -2 step penalty to his thread weaving talent to do so. Third, he makes the Spellcasting test, also with a -2 step penalty. Finally, he determines effect normally.
A mage using raw magic draws the magic of astral space directly through his body to cast a spell, instead of filtering it through a pattern or matrix. Most of astral space in Barsaive has been polluted by the Scourge, so this usually damages the mage trying it. In addition, it sets off a huge astral flare that can draw the attention of Horrors. However, the time may come when a mage must choose between the risks of raw magic and even greater risks of not using it.
The effects of raw magic will vary with the level of corruption of astral space where it is used. Safe areas are those where no Horrors have entered astral space, such as Throal and Travar. Open areas have seen little magical activity from the Horrors, and have suffered minimal destruction, which includes most of Barsaive. Tainted areas have seen extensive activity and destruction from Horrors, such as most destroyed kaers and citadels. Corrupt areas are currently inhabited by Horrors, and include all of the Wastes and the Badlands, as well as parts of Parlainth.
Aside from not bothering to attune a matrix, casting a spell with raw magic is no different from normal matrix casting. After a mage successfully casts a raw magic spell, the GM makes one to three tests. The first is the Warping test. Astral space will attempt to warp the mage, using a step that varies with the level of corruption and the circle of the spell. This test is against the base Spell Defense of the mage, unmodified by any magic objects or permanent threads that would normally protect him. Then it makes a damage test, again based upon the circle of the spell and how corrupt the area is. Finally, if there is a Horror nearby with Astral Perception and spellcasting, it can try to place a Horror Mark on the mage's True Pattern. A Horror Mark allows the Horror that placed it to bypass the mage's magical defenses, and use its magic on him at great distances, as well as communicate with him telepathically. They are very hard to spot, and last for a year and day, though the Horror may renew them if it wishes.
The Horror Mark Step cannot exceed the Horror's own normal spellcasting step. Circle always refers to the circle of the spell being cast with raw magic.