The use of Taint Spell allows the character to warp the essential nature of another spell, bending it more towards evil 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 "Taint Spell" as possible selections. Taint Spell is therefore similar in usage to skills like Smite and Focused Attack and combat weapons.
Taint Spell costs 2 Action Points and a number of Magic Points equal to 75% of the MP cost of the spell to be Tainted (rounded up). This means that while the AP cost is raised, the MP cost is reduced. The Tainted spell has its damage type changed to Unholy (yet still retains Affinity bonuses), and will deal additional damage to targets of positive Morality, equal to their morality divided by 10 (rounded up). The damage is further increased by 2 if the target is an Angel.
If Taint Spell is used against a Stronghold Ward, the MP cost reduction applies but the AP increase does not.
A Defiler knows a hypothetical spell "Ice Storm" which deals 16 Cold damage and costs 9 Magic Points to cast. The Defiler decides to use Taint Spell along with Ice Storm when attacking a Paladin with 29 Morality. Tainted 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 21 points (Ice Storm's base of 16 points +3 due to the Paladin's Morality and +2 for an attack against an Angel) of Unholy damage.
If the target had been a Myrmidon with a Morality of 12, then the attack would have dealt 18 points of Unholy damage. If the Myrmidon had a Morality of -2 then the attack would have dealt 16 points of Unholy damage.
Compare: Sanctify Spell