Shadowform

Assume a Shadowform, increasing your Shadow damage by 15% [S03 - Tuning and Overrides Passive - Priest: reducing the mana cost of all Shadow spells by 0%] and reducing Physical damage done to you by 15%. However, you may not cast Holy spells while in this form.

Assume Shadowform, increasing your Shadow damage by 10%, reducing the Mana cost of all Shadow spells by 50%, increasing the critical strike damage bonus of your Shadow spells by 100%, and reducing Physical damage taken by you by 15%. However, you may not cast healing spells while in this form.