From WoWWiki

Shadow Priests draw deep from the shadows to slake their thirst for power. They're never thirsty for long.

Shadowform
Shadowform Instant Cast Self range 32% Base Mana Assume a Shadowform, increasing your Shadow damage by 15% and reducing Physical damage done to you by 15%. However, you may not cast Holy spells while in this form.
Usable By
Class	Priest
Properties
Type	Offensive
School	Shadow
Cooldown	2 sec
Talents
Talented	Yes
Talent Tree	Shadow, Tier 7
Talent Required	Vampiric Embrace
Other Information
Ranks	1
Related Buff
Shadowform Shadow damage you deal increased by 15%. Physical damage you take reduced by 15%. You may not cast Holy spells. Duration: n/a

Shadowform is a Priest buff that can be acquired from the Shadow Magic talent tree. While active, it increases Shadow damage dealt by 15% and reduces physical damage taken by 15%.

Contents 1 Notes 2 Talent improvement 3 Tips and tactics 4 Known bugs 5 Past changes

[edit] Notes

• Shadowform does not lock the Holy school, but only the Holy tree. That is the reason why Holy spells from the Discipline tree, such as Power Word: Shield, can still be cast without removing Shadowform.
• The character looks translucent black while in Shadowform. While mounted, your mount also gains this appearance.
• Unlike Darkness, Shadowform does affect Shadow spells from the Discipline tree (e.g. Mana Burn) and shadow-elemental wands.
• Normally a Priest who uses this spell is considered a Shadow Priest.

[edit] Talent improvement

Mental Agility reduces the mana cost of all instant-cast Priest spells, including Shadowform.

[edit] Tips and tactics

• Shadowform's damage reduction is before the damage reduction.of Armor. If a melee blow would land for 1000 damage on an unarmored opponent, shadowform would reduce it to 850 before armor reduction is calculated. Shadowform does not *stack* with armor. See field tests here: http://forums.worldofwarcraft.com/thread.html?topicId=4463777931&sid=1&pageNo=2.

[edit] 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.

[edit] Past changes

• As of patch 2.1.0, Shadowform now has its own action-bar, like a Rogue's stealth, Druids' various shapeshifting forms, and Warrior stances.

Playable Classes
Classes	Death knight • Druid • Hunter • Mage • Paladin • Priest • Rogue • Shaman • Warlock • Warrior
Priest	Priest races • Priest quests • Priest abilities • Priest trainers • Priest talents • Priest builds • Priest tactics • Priest sets
	Starting a Priest • Working with a... (Priest) • How to kill a... (Priest) • Priest macros • Priest category