To play a caster well you need to be able to customise your spell book for the opponents you’ll face. To do this you use metamagic feats. These feats allow you to take spells at different levels other than their native level with added effects, and so allow you greater flexibility to match spells to opposing resistances and saving throws.
There are four metamagic feats that allow you to customise a spellbook: Extend Spell, Silent Spell, Empower Spell, and Maximise Spell.
Extend Spell – What it does
Extend spell doubles the duration of any spell with a duration (as opposed to being an instant, one-off effect). Extended spells occupy one spell slot higher than normal.
Extend spell is usually used with buffs, though can be cast on attack spells with a duration, such as Creeping Doom and the bigby spells. If a spell lasts ‘one round per caster level’, then a caster with 40 caster class levels will have that spell last 40 rounds, or four minutes of real time. This is not long for a buff, so a caster may choose to take the extend spell version of the buff at a higher level.
When should you use Extend Spell?
Extend Spell is used both to reduce the requirement to constantly recast buffs, as well as to free up lower level spell slots. For example, the level 8 cleric spell Aura Against Alignment lasts for one round per level. Taking this spell extended in level 9 allows it to last two rounds per level and frees up two level 8 spell slots which would otherwise be needed to get the same result. While level 9 slots are more powerful, one level 9 for two level 8 slots is a good trade.
Silent Spell – What it does
Silent Spell allows you to cast any spell while under the effect of the silence spell. More importantly, it lets any spell be cast at one slot higher than normal.
When should you use Silent Spell?
Divine casters who specialise in Drown or Harm can use Silent Spell to increase their amount of these spells. A cleric with Master of Dark can use Silent Spell to take Harm in both level 6 and level 7. Likewise a druid with Master of Force can use Silent Spell to take Drown in both level 6 and level 7.
Empower Spell – What it does
Empower Spell adds +50% to variable damage effects. 2D6 becomes 2D6 multiplied by 1.5.
A slightly above-average damage roll with Empower Spell will lead to a result equal to the maximum roll. Higher rolls will lead to results over what would otherwise be the maximum.
For example, the maximum of 2D6 is 12. However, if 8 is multiplied by 1.5, the result is 12. 8 is only slightly above the average 2D6 roll of a 7. If the 2D6 role is 10, then the spell will inflict 15 damage.
Empowered spells occupy spell slots two levels higher than normal.
When should you use Empower Spell?
The added damage and two slots higher effects of Empower Spell make it one of the most valuable metamagic feats for any caster build.
Typical spells empowered are the main attack spells of Blade Barrier, Ice Storm, Fire Storm, and Isaac’s Greater Missile Storm.
Because Empower Spell makes a spell occupy a slot two levels higher than normal, it is useful for filling mid-level gaps in spell books that casters often encounter. For example, empowered Blade Barrier is better than most anything else that can go in cleric level 8, even the innate choices. The Master of Force mythic feat just reinforces this.
Empowered Ice Storm will do more damage than many other level 6 choices for mages, and empowered Isaac’s Greater Missile Storm is a strong level 8 direct attack spell.
Maximise Spell – What it does
Maximise Spell allows a spell with variable damage effects to always take effect at the maximum result possible. A spell that does 2D6 damage will do 12 damage, for example.
Maximised spells occupy spell slots three levels higher than normal.
When should you use Maximise Spell?
Maximise Spell is very similar to Empower Spell but gives flexibility to take spells at three levels higher. This has to be balanced against your stronger innate spell choices as spell levels increase, particularly when taking level 6 spells at level 9, as level 9 for each caster contains uniquely powerful spells.
For example, while Empower Spell is very useful to fill the cleric level 8 spell gap with Blade Barrier, the cleric has Mass Harm and Storm of Vengeance at level 9, so maximised Blade Barrier is not always the best choice. Similarly, arcane casters have Mordenkainen’s Disjunction, Bigby's Crushing Hand, Dominate Monster, and Timestop.
Due to this, maximised Ice Storm (druid level 8, mage level 7) is one of the strongest uses of Maximise Spell, especially when combined with empowered Ice Storm, as it does not replace a level 9 spell. However, maximised level 6 spells are still useful in certain situations. Maximised Blade Barrier, Undead to Death and Isaac's Greater Missile Storm are worth having access to.