Spell Notes

Some spells are described pretty tersely in the manual, and this page tries to give a bit more info about some of the less straightforward aspects of the spells.
It does not give the specific parameters of the spells; Flamestryke's site (see links section) provides every spell parameter you want to know; and the spell mastery page shows exactly which class can cast which spell, and up to which mastery level.

The spell list below contains references to the manual; this is the acrobat reader (pdf) file on the CD; not the flimsy player manual in the box. (Even though the manual leaves a number of gaps, it at least contains some useful info too.)

In the list below I differentiate between skill level and mastery level: with skill level I mean the number of times the skill is increased (perhaps augmented by items), while mastery level means whether the caster is normal, expert, master or grandmaster in that skill. If a spell mentions some extra effect from e.g. Master level onward, it means that the caster needs to master or grandmaster in the spell's primary skill.

About resistances: almost any offensive spell is from elemental or dark magic; the only spell that checks against light resistance is 'spell reaver', while spirit resistance is only 'useful' against turn undead :)
When looking at the attack types of monsters, physical attacks, dark and elemental are common, as expected. Light attacks are also very rare (only monk-types), but many of the tougher monsters use spirit attacks, like the beholder family (evil eyes etc.), making spirit resistance a lot more useful than you would think.
Physical resistance is less clear: the manual says it helps against 'magically- caused Physical-based attacks'. So you would expect it wouldn't help against swords or claws. But later in the game some opponents get less damaged by your physical attacks than the numbers suggest, which would suggest that physical resistance helps against all physical-based attacks. By the way, only a few artifacts can increase this resistance for your characters.

This list is not objective; I've tried not to color it too much by my own perception, and to express general opinions, but most of the information below is debatable.

General Spell Remarks

Arms of Earth
You can forget about the warning in the manual about being caught in your own trap.
Cheap spell that might make the difference. If you miss often, this spell might double your successful hits. (Targets the whole party at any level.) You can counter a curse spell with it, too.
Chain Lightning
The damage this spell does, multiplies by the number of opponents in range. One of the most valuable multi-target spells, not in the least because it has an extremely short recovery time for a GrandMaster Elemental.
Has a small area of effect; mostly useful against creatures that are very vulnerable to fire, and to little else.
It's effects increase dramatically with your Dark mastery: while at normal it only decreases the to hit of the single target, at GrandMaster level it not only decreases the to hit almost 6 times as much, but it also halves the attack rate, and prevents spellcasting on every monster on the screen.
Backside is that it has a very short duration.
Dark Grasp
Decreases target's effectiveness fairly good, but it's not used often.
Usually just dealing damage seems more effective, and even at GrandMaster level Dark this spell only targets a single opponent.
Death's Touch
About as cheap as it's useful.
This spell's casting cost rises with higher elemental skill, without getting any more effective! It does a fair amount of damage, eventually, but are you going to wait for it? Perhaps nice in combination with turn undead, but even then...
Divine Intervention
Life saver.
Elemental Aura
This spell seems to function different from it's description: it does prevent damage, but doesn't return any. Nice thing about this spell is that it lasts until the shield's hitpoints are gone, although it will disappear eventually (24 hours?).
Elemental Blast
Get it as soon as you're expert elemental and light. Damage stays comparable to higher level spells, like column of fire.
Elemental Bolt
Will stay your quickspell for quite a while. Cheap and reasonably effective.
Elemental Protection
Targets whole party from Master level Elemental onward.
Enchant Item
Enchant every 'bare' item you find, and you'll earn quite a bit more money.
Note that it's chance of success decreases when you become expert Spirit, before increasing again on becoming master.
It's fun to watch opponents kill of each other, and can change the odds in large fights. Has a short duration though.
Eye of Leggib
You can use this spell to take a peek around a corner or through a closed door, with little chance of discovery. (Stealth increases with Dark skill mastery.)
Be quick: it's over again before you know it.
Eye of the Storm
This spell can only be cast once per 24 hours, has a radius of only 10 feet around the caster, and grows stronger with more opponents in the area. But when you cast it right, no other spell matches it's damage.
Adds a fixed amount of damage to one attack (even when you have multiple attacks, you only get the bonus once). Adds to melee and missile attacks.
Targets the whole party from Master level Light onward.
Useful to split up big groups and kill them one by one.
Feather Fall
No damage from falling; one of the few spells useful for a paladin or ranger: only the duration increases with higher level, but you usually cast it for 1 fall anyway.
Fleet Foot
Warning: this spell is addicting! R u n n i n g seems so slow again without it :)
No weakness afterwards. Speeds up combat a fair amount at higher levels.
Targets the whole party from Master level Light onward.
You might have discovered the use of this one by yourself. Gets a lot more effective again at GM level Spirit.
Lloyd's Beacon
Cast at Master level Elemental, allows up to 3 beacons; at GM level up to 5. The beacons are shared with the whole party, so one character can return to a beacon set by another. This also means you can never have more than 5 beacons at a time though.
The spell description reports an inaccurate duration of 1 week, the data files seem to suggest it lasts 1 week per skill level Elemental, and expert players claim their beacons never expire. I'm tempted to believe the last.
Masters Elemental can cast the spell when the monster alertness gem is yellow, GrandMasters even when it's red (in melee).
Magic Mine
This spell sounds pretty good: it should last until triggered, and according to the data files, should do (12 to 20)x(elemental skill)/3 damage on grandmaster, for an average of 80 damage when you have a skill of 15. In practice the damage is very low: 5 + 0.6x(elemental skill) at grandmaster level, so with an elemental skill of 15, you'll only do 14 damage, per mine. Next to that, it will explode after a minute, even when there are no monsters near, so you can't make traps with them to lure monsters to.
All in all: totally worthless.
Meteor Shower
In contrast to earlier Might and Magics, I have never succeeded in harming my own party with this spell. In further contrast, it's way less effective too.
Natural Armor
The manual says: caster cannot be wearing metal armor; the in-game spell description says the target cannot be wearing metal armor; but in practice, both are allowed. This makes it a fairly decent spell.
Pain Reflection
Works great (as long as the monsters hit you), even against missiles. At Grandmaster level Dark the whole party is targeted.
Works well, though short. Tougher monsters fairly often resist the spell though.
Phantom Fighter
You need one-handed, bladed weapons (swords, axes) for this, unenchanted for expert level, can be enchanted on (grand)master level Spirit. At expert level, you need pretty crappy weapons for the spell to work; on grandmaster most blades will do. It's supposed to use the magic properties of the sacrificed weapon, if that has an enchantment.
If no monsters are near, the (disembodied) floating weapon will patrol a bit.
And if you finally succeeded in casting it successfully, and sacrificed a weapon to it, you get a fighter that is of fairly little use to you. See here what two axe-phantom fighters look like.
Same direct damage as elemental bolt, but adds 1-2 damage for 10 turns, for around double the spell point cost.
Does get a radius from Elemental Master level onward, in contrast to it's description.
Poison Cloud
One of the strongest single-opponent damaging spells. Damage seems only slightly higher than other spells, but because an opponent receives the damage several times while the cloud drifts by, it is very strong. You might catch a second opponent behind the first, but it must be straight behind the first for that. You can even improve the damage done by aiming the poison cloud partly up or down.
Power Draw
The spell description suggests you can decide how much spellpoints to use, but it has a fixed casting cost. Furthermore I can only cast it successfully on NPCs; and afterwards I don't see obvious improvement in damage, nor damage bonuses.
Another one for the dustbin.
Disease from piles of rubble and corpses can only be cured from Light Master level onward.
Never regenerates more than 1 hp per minute (although reported working fine before (official) patch 1.2).
Resist Death
Targets the whole party at Grandmaster level Light.
Later in the game, deaths happen more often, because the opponents do enough damage to go from conscious to dead in one blow. At grandmaster level Light, the ex-dead can immediately join the fight again, though weak.
Shared Life
Nice in the earlier phases, where you can siphon off hitpoints from NPCs this way.
Doesn't work on friend, only foe. Healing and damage all in one, and not even that expensive! Most undead have very high dark resistance, though.
The first multi-opponent damage spell you can get. It's pretty erratic though, and not a top damaging spell.
Spell Reaver
Can't think of any spell I'd want to remove from an opponent: no monster casts defensive spells, and you can't use it to remove your own negative enchantments, like dark grasp. (Curse can be countered by bless.)
Doesn't have a very large radius, but helps to spot treasure bags in dark corridors. For other situations you can better increase the gamma correction (under Video Options).
Town Portal
Teleports to any town with a cleansed altar from Master level Elemental onward, but on expert only to the nearest.
Experts and Masters Elemental can cast the spell only the monster alertness gem is green, GrandMasters even when it's yellow. For experts the spell will fizzle when cast in a town with a (cleansed?) altar.
Automatically targets an opponent when there is one, or fizzles. So how to use this spell against (?) a party member is unclear to me. The effect is kind of hard to judge too. (haven't found a good use for this spell yet)
Turn Undead
Works better than terror, because turned undead keep trying to get away from you even when you damage them. From Master level onward all undead visible are effected, and on Grandmaster it even does considerable damage. Not having a grandmaster spirit in your party is definitely a handicap!
Wizard Eye
Some do without this spell; but if you get used to it, you'll miss it when it expires: it's nice to see where for instance those dragonflies are, and also useful to spot treasure bags or chests you missed. Shows those from Master level onward.
Another single opponent damage spell. Never used it much; didn't seem as effective as other spells.
Wrath of Bugs!
Makes monsters attack slower. I usually prefer curse or dark grasp, if I use a disabling spell at all.