So if you're a spellcaster of some sort, you'll probably be wondering what do with all of the possibilities. Well, it's actually fairly simple when you think of it. Spells are divided in three ways: Buffing/Support spells, Glyph Spells, and damage spells.
Damage spells can be divided into a three categories: Autohit, Ranged, and Touch. Furthermore, there is one of each spell for each tier, for a total of six spells of that particular damage type. Autohit spells are just that: they have low damage and fairly high cost, but are always guaranteed to hit and do damage. Ranged spells use Spell Combat accuracy and have higher damage. The tier 2 Touch spell has decent damage, uses Hand to Hand Combat or Melee Combat or 70% accuracy, whichever is highest. The tier 3 "touch" spell is an aura that can also be used to supplement the damage of the tier 2 touch spell of that type as well as provide a defensive damage aura against any attacks that make physical contact against you. Tier 2 spells all cost 10 CP each, while the Tier 3 attack spells cost 15 CP and the aura costs 20 CP. Thus it requires 80 CP to learn all the spells of a particular damage type.
Because of this, it's generally rare to see a spellcaster with more than one or two full spell trees unless they are a dedicated spellcaster as any more will considerably cut into valuable T3 skills or just as valuable support spells. However, a spellcaster's T3 class choice will have a strong effect on their choices in spells. Wizards might prefer complete spell trees, while enchanters regardless of class will want to have a spread of damage types, for example.
Despite the fact that there's essentially three different types of combat spell requiring three different trees, most characters will try to acquire all spells of that damage type for the "Mastery" bonus. Each spell beyond the first you know of that damage type increases the damage for all spells of that type by 1 to a maximum of 5, with the exception of auras, which have their duration increased by that number of ticks. Furthermore, Tier 3 attack spells gain a "Capstone" bonus, a unique bonus provided by that damage type.
Damage types for Dummies
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There's seven types of damage spells available, although one of those damage types will be restricted from Angels and Demons. Transcended casters are not limited in their spell selection. In general your choice of damage type will be dependent on your class, but also what you expect to fight: if you're going to fight a lot of demons, taking fire or acid might not be the best of choices. Furthermore, while Tier 2 spells and auras
Acid: Angel-restricted. This is a good damage type that not a lot of non-demons resist well. If you can engineer them to have 0 soak or less, Capstone Acid can become quite devastating. Because of this Cold is a common pairing with Acid. If your primary targets are demons other damage types are superior.
Cold: Popular for the soak reduction capstone and the fact that it's not a super common soak, cold is the classic anti-demon and anti-angel damage type. Also popular paired with Acid for it's capstone. Because of the soak reduction, T3 Cold Capstone can be invaluable for petmasters.
Death: An uncommon soak with a couple of very glaring holes. Soul Fray is a useful capstone effect, which can save a good bit of MP, but Death Mantle is useless against Revenants, Liches, and Lich pets, all of which can be quite dangerous.
Electric: Demon-restricted. Electric Capstone will save you MP by lowering target defense, making further spells more likely to land. For the same reason, Electric spells also make excellent petmaster support. Between that and Electric being an uncommon elemental soak, Electric is a solid choice.
Fire: Fire Capstone is useful against doors and forts but it's a common mundane soak, which makes fire less useful during tier 2. Tier 3 spells are better but still suffer against player characters, so it's not a common choice.
Impact: Capstone bonus is great, improving the effectiveness of T3 damage spells, but Impact is a very common mundane soak, which makes up for that.
Piercing: Capstone is basically Impact, only relevant if the target has at least 3 soak. Like Impact, it's a common mundane soak, so Impact/Piercing tends to be a roughly even choice.
Slashing: Another common mundane soak, with an unreliable Capstone. Of the three "Physical" spell types, this is the weakest, although the +5 damage surprise is often funny when it triggers.
Arcane/Holy/Unholy: Restricted to those with Deep Spellcraft, Sanctify Spell and Taint Spell skills. All three "aligned" soak Capstones add damage in different ways: Holy and Unholy is dependant on target type and morality, while Arcane destroys target MP for bonus damage. Specialized and takes additional effort (MP/MO/HP, depending on skill) but usually worthwhile. As there is no direct "spell tree", the Capstones require using a spell damage type with a Capstone bonus in a charge attack provided by that class' particular skill, effectively costing 40 CP to use once you have fully learned a spell tree.
Support spells
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In general, you're going to want to set aside maybe 20-30 CP for buffing spells. 10 CP for the spell that improves the accuracy of your preferred combat tree (Sharp Vision for Ranged spells, Tiger Claw for touch+aura spell combo *providing you use a combat tree and not base 70% accuracy*). Reveal and Sixth Sense to find targets, Levitation for mobility, Conjure Sinews for utility/physical support, or Mystic Shield, Mystic Mail or Blur for defense. These spells are also good ways to round out Tier CP requirements. Setting aside 50 CP for spells during T2 (All T2 attack spells, accuracy spell, other support) is not a bad idea. There's also Shadow Wrap which can be a big survivability boost if you make sure to save some MP for the end of your cycle and perform good hiding practices. If for some reason you decide to pick up Advanced Hide, you get a weird kind of charged attack that costs 1 AP and 5 MP for an accuracy boost that you probably shouldn't be *that* desperate for.
Glyph spells
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Glyph spells are the other primary type of spell available and involves placing zones that last for a base 2.5 hours (150 minutes) that affect characters of a particular alignment in one of four ways. Glyphs of Protection prevent passage almost entirely, while Glyphs of Pain, Sapping, and Slowing drain HP/MP/AP respectively for each AP used. Greater Glyph Erasure does impressive damage to Glyphs, destroying most glyphs in one or two castings. The Glyph spells are very specialized spells that are best used for factional support.