Spell difficulty classes in WotR
A spell's difficulty class, or DC, is the saving-throw target. The usual calculation is 10 plus spell level plus the casting ability modifier, followed by applicable bonuses.
Spell level and casting ability
A level 3 spell with a +5 casting ability modifier starts at DC 18 before other bonuses. Caster level and spell level are different values: raising caster level does not automatically add the same amount to DC.
Metamagic that only raises the spell slot cost does not automatically raise DC. 
Heighten Spell can increase the effective spell level used by this calculation. Check the particular spell and its applicable bonuses.
Heighten Spell raises the effective spell level used by saving-throw DCs.
Standalone and merged mythic spellbooks
A standalone mythic spellbook uses mythic rank instead of the casting ability modifier in this calculation. For a level 3 spell at mythic rank 5, that gives DC 18 before other bonuses.
A merged mythic spellbook uses the ordinary casting-stat branch. It does not receive the standalone mythic-rank substitution merely because it includes mythic spells.
Some abilities replace their parameters, and item-cast spells can have their own DC. The ordinary formula does not override those rules.
Enemy difficulty adjustments
For an enemy ability using this calculation, difficulty adds the following DC adjustment.
| Preset | Enemy ability DC adjustment |
|---|---|
| Story | −4 |
| Casual | −4 |
| Normal | −2 |
| Daring | −2 |
| Core | 0 |
| Hard | +2 |
| Unfair | +4 |
These are separate from enemy ability-score adjustments. A change to the caster's ability modifier may also affect its DC before this addition.
Some enemy abilities soften high DCs
Only abilities that carry the difficulty-limiting rule use this additional reduction. On Story, a DC above 14 keeps one quarter of the excess; on Casual, the threshold is 18 with the same fraction. Normal uses a threshold of 22 and keeps half the excess. Fractional excess is rounded down.
For example, an affected Normal-difficulty ability at DC 30 after its difficulty bonus becomes 22 + floor((30 − 22) / 2) = 26. It is not capped at 22. Daring and higher do not impose this reduction through their presets.
Changing only damage to the party does not change this saving-throw target.