It is one of three possible abilities gained from the quest "Black Book: The Sallow Regent."
- The 10% cost reduction only works on base cost of the spells. i.e. Spell with 100 Magicka only needs 90 Magicka to cast. The effect doesn't stack with cost reduction from enchanted equipment. So even with 90% cost reduction equipment, they will reduce the cost to 9 Magicka instead of 0.