Type: Imbuement
Strategy: The aid effect is nice, but the additional 1d8 hit points are pretty much negligible to anyone but a mage. It is advised that you simply stay with bless, which is one spell rank lower.
Importance: Low
Type: Defense
Strategy: With effects that increase as you level up, barkskin begins useful and stays useful. Eventually lowering your base armor class to two, this spell will usually be one step ahead of your ability to find better equipment. It's definitely one of the better lower-level armor spells.
Importance: High
Type: Imbuement, hindrance
Strategy: Raising all party rolls by one, and lowering all enemy rolls by one, chant is simple but pretty effective for a level 2 spell, having the same general effectiveness as a double-bless spell.
Importance: Moderate
Type: Charm - druid only
Strategy: This spell grants control of any one humanoid or mammal enemy to the player for five rounds. Toward the end of the original Baldur's Gate, most creatures resisted the charm series of spells. The same problem is present here in Baldur's Gate II, so most often, the spell will never get off successfully. However, keeping one copy of the spell memorized is advised for earlier portions of the game for those times when you do run into a person or mammal susceptible to charm.
Importance: Moderate
Type: Imbuement
Strategy: Gaining power as you level up, this spell will increase strength, constitution, and dexterity by one point for every three levels of the caster. In the later portions of the game, this spell can take your stats to godlike numbers and will give substantial bonuses. As you progress, this spell becomes increasingly important and will play a key role in making your priest one of your best all-around party members.
Importance: Vital
Type: Other
Strategy: Just like the thief's ability, this spell will let you locate traps. If you have a thief in your party (and you should), this spell is useless. If you don't, this spell is a must for dungeons.
Importance: Low or high, depending on party makeup
Type: Summoning
Strategy: This spell summons a blade of flame (1d4+4) for the caster to use; this spell is especially good against trolls. As its name suggests, the blade is made of flame and will hence kill trolls. Otherwise, its damage and bonuses against the undead are actually quite nice and may be better than the weapon you have for the first bits of the game.
Importance: Moderate
Type: Cure, heal, and summoning - druid only
Strategy: Healing one point of damage, goodberries aren't so tasty (excuse the pun). Truly one point of damage is as inconsequential as it gets. Don't bother with this spell.
Importance: None
Type: Hindrance
Strategy: This spell freezes one humanoid creature in place, letting you do anything to it without consequence. Early in the game, when the spell is not resisted, it can work wonders again humanoid enemies. Later in the game, its use tapers off.
Importance: Moderate
Type: Other
Strategy: If you ever find yourself in a situation where you are not sure whether to trust or not to trust those with whom you are dealing, casting this spell will answer it. However, it is usually more beneficial to follow through and trust, even if you know it's a trap, as the result can lead to hefty treasure, experience, and even new party members.
Importance: Low
Type: Defense
Strategy: If you find yourself under cold or fire attacks from spells, traps, or anything else, casting this will absorb 50 percent of all cold and fire damage. This happens very infrequently in Baldur's Gate II and is only usable under select circumstances.
Importance: Low
Type: Hindrance
Strategy: Casting silence will prevent enemy spellcasters from casting. A most efficient spell, but it's resisted most of the time. If you are not too far into the game, keep a copy memorized and see if it will affect anyone. If so, the cast will be well worth your time.
Importance: Moderate
Type: Cure, Heal
Strategy: In Baldur's Gate II, most poisons will require a more potent spell to neutralize the poison. However, this spell will slow the poison. Use it only as a backup if all other poison cures are unavailable.
Importance: Low
Type: Summoning
Strategy: Summoning a magical hammer that will be +2 or +3, it is entirely up to you whether it is worth it or not. If your weapon is not as good, casting spiritual hammer will help. If your weapon is better, there is no point. You will most likely have a better weapon soon enough.
Importance: Low