The use of Sanctify Spell allows the character to temper the essential nature of another spell, tuning it more towards good ends. It can only be used with Offensive Spells the character has learned (so it cannot be used with spells cast from a Spellgem, for example and will not activate with Aura Spells).
The player will see a new menu next to the spell selection menu after this skill is purchased. The menu will have the options "None" and "Sanctify Spell" as possible selections. Sanctify Spell is therefore similar in usage to skills like Smite and Focused Attack and combat weapons.
Sanctify Spell costs 2 Action Points and a number of Magic Points equal to 75% of the MP cost of the spell to be Sanctified (rounded up). This means that while the AP cost is raised, the MP cost is reduced. The Sanctified spell has its damage type changed to Holy (yet still retains Affinity bonuses), and will deal additional damage to targets of negative Morality, equal to their morality divided by -10 (rounded up). The damage is further increased by 3 if the target is an Demon; however, the damage is reduced by 1 if the target is not a Demon.
If Sanctify Spell is used against a Stronghold Ward, the MP cost reduction applies but the AP increase does not.
A Shepherd knows the hypothetical spell "Ice Storm" which deals 16 Cold damage and costs 9 Magic Points to cast. The Shepherd decides to use Sanctify Spell along with Ice Storm when attacking a Pariah with -40 Morality. Sanctified Ice Storm costs 2 AP and (9 MP x .75% = 6.75 rounded up to 7) 7 MP to cast. A successful hit will cause 23 points (Ice Storm's base of 16 points plus +4 due to the Pariah's Morality and +3 for an attack against a Demon) of Holy damage.
If the target had been a Myrmidon with a Morality of -12, then the attack would have dealt 17 points of Holy damage. If the Myrmidon had had a Morality of 2 then the attack would have dealt 15 points of Holy damage.
Compare: Taint Spell