Shadowform

Shadowform is the Tier 3 Shadow priest talent ability. It causes the priest to assume a Shadowform, increasing Shadow damage dealt by 15%, reducing all damage taken by 15% and increasing all party and raid members spell haste by 5%. However priests in Shadowform are unable to cast spells from the Holy tree, limiting their options and further emphasising the separation between Shadow priests and their Holy brethren.

Talent improvement

 * Mental Agility reduces the mana cost of all instant-cast Priest spells, including Shadowform, by 3/6/10%.

Known bugs
Look-altering actions that take place while shadowformed (such as being poisoned or diseased) will alter the look until Shadowform is dropped and reassumed.